I think I wouldn't touch this question with the proverbial ten-foot pole! No good can come of answering a question like this. For one thing, you only have one side of the story. And, while this may be the way your friend was feeling when he asked the question, this does not mean he always feels this way. You also don't know what the circumstances are when his mental health problems manifest themselves. And you don't know how it is his mental health problems are manifesting presumably, because you're not always there. Also what if, in the heat of battle, your friend blurts out to his partner: "Well, I talked with (Cygnus) & s/he said I should dump you too!"

The only answer I can think of that might be appropriate would be: "Perhaps this is something you could talk over with your therapist" (or "a therapist" if he doesn't have one.) Anyway... my recommendation is to steer clear of this question!