Last line is mocking in nature.
Now, coming to a line about woman who fell for her friend's partner sacrificing him for the same friend, it's not a sacrifice when things have gone too far. Everything has a right time, even sacrifice has a right time. Right time for Nandini was when she was leaving the city at that time when she had just found that her feelings for Kunal won't let her live near him.
This time sacrificing doesn't make sense when she has not done the closure. Just asking the person to go & never call again isn't a closure. Immature people do that. Mature people despite terrible pain, break up with a lot of consideration & talk. If Nandini had to be shown strong then that is the route makers should have gone. Her leaving abruptly will just madden Kunal all the more. So, no, this wasn't a sacrifice. It's not a sacrifice if you left him with some kind of hope to linger on.
I can give you countless examples of people actually sacrificing their own love for the betterment of other person.
I don't know why do people feel, Mauli is unrealistic character, when in reality she happens to be very real character. I have seen women like Mauli in my own family very closely. They are real.
If Kunal & Nandini are real & grey, then Mauli too is very much real.
In fact when it comes to brutality, Rajdeep too is real since I have heard real cases where one can't even imagine the evil, a human being is capable of doing. (Just recently, a woman murdered her husband, burnt her face with acid & destroyed his body, then she went on to give her husband's face to her boyfriend with whom she was having an affair through plastic surgery. This is a case reported from Tamil Nadu quite some time ago). And there are countless example that will leave one shocked beyond imagination.
And yes, when couple don't have communication about gravest of issues & go on pretending that they are like normal couple, then it is likely to be perceived very differently.
Nandini was an abuse victim, she was raped when she was in relationship with Kunal & still Kunal-Nandini never even tried to talk about it. Kunal never even dared to understand Nandini beyond her looks & cooking skills. Nandini never tries to talk to Kunal about their situation, about a wife at home even when she knows or about the future.
It's absurd to compare Kunal-Nandini with our parents, who liked their spouse with the picture and not fall in love immediately. Falling in love was a journey for them which took time unlike here, where a Baba prediction makes Nandini fall for Kunal so hard. You can call it modern love & spare the old ways of falling in love from comparison. Our parents were not in love with someone else already when they took to liking pictures (most of them had arranged marriages). They has clear minds, saw the picture & took to the liking. Kunal was perfectly in love with Mauli till the rain dance & it was only when he had encounter with Manas, he knew he had infact fallen for Nandini, until then he had guilt of having those thoughts.
Marriage doesn't fall in a day, it takes a lot of emotional cheating for it to fall down. If Kunal was sincere about his marriage, then he would have out more efforts, but he didn't. It's easy for him to say, I fell out of love just like that & fell for other person, but imagine it on other person who is still not there at any level. It's cheating even when it is happening in relationships other than marriage. Kunal didn't mutually break up with Mauli or himself reveal it to her, Mauli caught him red-handed cheating in open space.
Kunal fell in love in that moment, it didn't evolve after the dance. It began precisely at that moment. It was this moment which marked emotional cheating on his part. And from here, there was no turning back.
If Mauli-Kunal marriage was unrealistic, then KuNan is way more unrealistic, since they don't have any emotional bases to their relationship. They didn't have emotional bond, it was all physical as they showed it. And they took it from emotional cheating to physical cheating so soon without even having any introspection. Even in boldest of EMA stories, introspection is there of the actions, guilt is there when facing the spouse on part of sensible people who realize they are cheating. Only insensitive people wouldn't show any such fight of emotions in such situations.
If Kunal-Mauli never talked about their differences, then even KuNan never talked about their differences, hell, they didn't even talk about dark things that each of them brought in the relationship.
Atleast Kunal-Mauli talked about differences in the beginning, while all KuNan do is talk lovey-dovey & talk about kheer & saree.
If Kunal had to assure Mauli, then he still has to assure Nandini, infact he has to assure Nandini way more than Mauli. It's for a fact from the show.
Mere rejection of Nandini without any reason made Kunal so mad & he did his best to try to make it right with her with video message, phone calls, visiting her home again & again, date, even though this relationship is merely a 1.5 month old. Now imagine, Mauli who has been married to this guy for 7 years, is expecting his child & this guy just informs her that the marriage is over, without even giving answer to her WHY?
Kunal too asked WHY from Nandini? Mauli too asked WHY from Kunal?
And none of them got answer to that which left them in such a mess within themselves.
Kunal still can't answer what went wrong in his marriage. Now can he? If he knew why he was going after Nandini, he would have known answer to this question.
Truth is Kunal himself doesn't know why he fell for Nandini at this point. Nandini knows why she loves Kunal because he happened to be a man who treats her with respect, but Kunal still doesn't know why exactly did he fall for her or why is so hell bent on rebelling. He just knows that he wants to live with Nandini because he thinks he loves her even when he doesn't know why exactly?
Kunal's inability to answer WHY? has left Mauli in hanging, since she isn't able to figure out what went wrong, when & why? This is a very haunting question for anyone who put their mind and soul to a relationship. And until they figure out answer to this question, they can't move on & live in peace.