Trust me, I think many of us would like to see Trishna treated well given everything she's been through. She's very considerate of Deep, and he usually is considerate of her, and very loving all.
The problem is as we saw, something in his past. And if something triggered a traumatic memory (music, scene/background, someone's look, words), it can be devastating and disconcerting. I don't think Deep treated her badly intentionally or that he didn't think about her. I just think he wanted to get away...by himself. He forgot everything else, and I can't hate him for this. And I'm not sure why I should? Because he's married and has a responsibility to be kind to Trishna and not be mentally ill???
With regard to mental illness, I don't think that applies here. He got a shock, something jolted a terrible memory from his childhood. Last time I checked, this would not meet the standard definition of any mental illness I know, except PTSD. But I can confidently say this was a bad memory. About your suggestion, just go out there and get help: Sometimes, mentally ill people are not able to "see", "observe", or "perceive" what is happening in reality. Their world is an alternative universe, and they are trapped in it, alone in it. Not comprehending why they are the way they are. Not understanding why they just can't snap out of it and see everything like everyone else does. It is a truly devastating situation, and it takes people years to overcome the short-circuit in their brains. It's just a serious thing and the casualness with which it was thrown around did surprise me.
A husband and wife have responsibilities to each other, and sometimes, due to situations out of anyone's control, you can't fulfill them. You can't fault in this situation. It just is. Be more understanding of relationships--it's just not about husband and wife.
You watch, Trishna will try to help Deep through this. And oneday when everything is resolved, he will respect and love her even more, and they will be closer.