Hmmm... With the higher end hotels, I would say that they won't have much problems booking them even if they aren't married. But if they're going to be booking cheaper hotels, I'd recommend just getting separate rooms since they aren't married. Not all cheaper hotels would have a problem with unmarried couples though, but just to be on the safe side, that's what I would recommend.
India is generally conservative but when it comes to the tourists, they don't meddle much as long as the unmarried couple acts appropriately. So there should be no PDAs, hugging or kissing at least in public places. And most especially on certain tourist destinations like temples, couples should be extra cautious and avoid being too sweet with each other.
I would also recommend booking in advance for a hotel. This way, they can be sure that a hotel can accommodate them despite them being unmarried.
I hope this helps!