Q:do you think he told me all this private to vent ?
A: Yes, it is possible that he is seeing you as his confidant. When a person is dealing with a lot of stress, it can to a certain extent impede their judgment. Should he have been in a better spot, he may have been more guarded and tactful.
You may want to remain supportive and at the same time to encourage him to solve these issues with his wife. Also, you do not want him to become dependent on you to purge his feelings, frustrations and pain. These are areas that would require a neutral and professional help because he himself has to find way of managing these in his life. Even though talking to you as his friend may alleviate some of the anxiety/stress it is temporarily. He would need to be proactive in helping himself and his situation both marital and in regard to his work/career. In that case, you may act as someone who encourages him to do so, try to limit your time spent listening to him not to foster dependency and learned helplessness. From what he had shared so far with you, use some examples of how if you were in his shoes you may try to help yourself, let him know that you'd want him to work on his well being and that you support self improvement. Try not to comment on the way his wife is or side with him and be more in the role of of the objective observer. Otherwise, he may interpret your statements/behavior to mean something else than what you intend it to be.
It was never suggested to you to quit on him as he is your friend; just to be supportive within limits. This man has to resolve his issues and even though they have gone to therapy, there are still other options. He may be subconsciously expecting that you fix his issues even if he does not necessarily directly express that. You've got to ask yourself what is he expecting out of his relationship with you? He is way over disclosing all of his baggage to you.
Even if there had been no overt indication, the mere fact that he bad mouths his marriage to you is a red flag.
I'm opting out of your question so another clinician can share their objective feedback. You asked if he had told you all of this to just vent and my reply to you is that it does not seem that way.