I think he knew that she was impressionable. He knew that he could manipulate her easily. And I think that he knew that she wasn't coping well, but he used that to his advantage. I don't think he knew just how serious Nina's situation was though.

Its hard to tell, but I think Thomas may have had the slightest idea about Nina's mental illness. We can't be sure what Erica said was wrong with Nina when she called to studio to tell them she wouldn't be coming to the performance, but when she came (and obvious wasn't physically ill) he must have suspected something. Then, at the end when she was laying on the mattress, he said, "What did you do?" instead of "What happened?" or "Who did this?" Still, I'm not entirely sure about it.