Arjun has the right to know about his child in Purvi's womb. But the situation is not in favour of that.
Purvi could have told the truth to Arjun before leaving the city. but it will turn against Arjun - Ovi relationship. Arjun will surely dump Ovi and run behind Purvi.
it is easy to blame Purvi's decision. when one sacrifice own happiness for others she/he will be blamed more and more even by the beneficier. but the pain undergone by sacrificier is unimagenable. we can understand only when we stand in her place. its fictional still there are people in every family doing some kind of sacrifice. unless mother sacrifices her own happiness her child won't be growing up happy.
Purvi still hiding the truth is fully acceptable. she wants Arjun to be happy with Ovi. after gifting it is not good to take back your gift. like that Purvi gifted Arjun to Ovi. now she can't take him back from Ovi.
Arjun intially suffered by Purvi's decision. now he too learnt to live for others happiness. he was worried about Purvi's future. now he is happy Purvi got good partner.
when he comes to know about his child, surely he will be upset on Purvi for hiding. at the same time he will stay calm for Ovi. till now we have never seen anyone doing anything which hurts Ovi. same will continue.
In reality, Onir-Purvi can raise the child. Arjun and Ovi can move on with their child. but its fiction. tears, screaming and shouting are part of the track. so the truth will come out. Ovi will continue to blame Purvi. Arjun will try to protect Purvi for his child. Aai and Baba all the time silent.
who knows Purvi will sacrifice her baby to Ovi and run out of the city again. Onir will turn negative to torture Purvi. Arjun will try to save Purvi from Onir. its all part of fiction.