I am still not onboard with rhea being planted in his life so I am going to stick by my own judgement. Simply by putting two and two together of things which we know for sure and there's no speculation.
Whether or not he was drugged cant be proved. Even if it makes sense, there's no proof. 
What we do know is that he was in his most vulnerable state. He was shown, made believe and instilled that he wasn't one of the them. He wasn't welcomed. He wasn't good enough. He was ridiculed to be in love with a girl much more high in status than him, that same girl let people make fun of him, that same girl swooned over other man every opportunity she got while being in a serious relationship. That same girl left him probably when his movie tanked and needed her the most. 
Rhea comes in and hits on all the right nerves to get into the good books. Façade or not, she did make him believe he was worth her love and care. Told him all the things he so desperately wished to hear. Pampered him with all the sugary compliments which he bdw never (not in videos atleast) reciprocated.. making him believe she's there. Making him believe she's the one. Making him believe in himself. There's this one interview in which he says he's happy when asked about his relationship status during chichore. His eyes and demeanour spoke a different story altogether though. For me. 
Just because she gave him all those feelings to feel which sara took away with her, he was too fast to let rhea into his life. Sometimes this happens when you so desperately wish to fix everything broken inside you to get over that sinking feeling. There's delusional feeling of being happy and being loved and to feel in love. I am not saying this is exactly what has happened. But from where i am seeing him, i feel this has a very high chance. She gave what he wanted and that probably made him so much in need of her to feel that kind of security about his existence. In attempt to hold on to the woman who accepted him for the kind of person and the kind of background he brought with him and also shared the middle class family history. No burden of royalty or big family name.
I was genuinely upset when i learnt he trusted rhea over his sister and that drugging him in that initial stage of relationship was a little too far fetched for me so i started rethinking his decisions and why would a guy choose a girl over his sister. 
Probably the pain initially and then the numbness towards it blinded his decisions. Emotional instability does make you extremely vulnerable and you end up taking steps without any rationalisation to just ease that pain. 
Please try to understand i am not playing rhea's advocate here. All i am saying is that even if he wasnt drugged, it was possible for him to give control of his life to someone he *wanted* to trust. 
Rhea was able to get into his life like this only because he let her. If there was any manipulation done, he couldnt see it because he didnt want to probably. 
So if now after so many revelations through so many people, I put sara in much worse light than any other person in his life, i mean it. Every destruction has its beginning and by far, she seems to be the beginning of his end. 
P.S : Anyone can choose to disagree. I by no means say these are the facts. They are not. This is just a perspective.