Because apparently I really need to redo all the 'horrible things happening to Matt' as 'horrible things happening to Foggy prompts'.

Matt can't exactly leave him on his own because who knows what side-effects it has, he also can't leave him with anybody else because he can't risk him spilling his secret.
So he has to stay. Knowing exactly that Foggy already isn't happy about how much Matt can tell about him with his senses and now he's spilling even more secrets.
Matt tries really desperately to get Foggy shut up/keep the conversation about non-incriminating topics but during that mentions something in passing that shocks him so much he's asked WHAT? before his brain has processed what a very bad idea that is.
Now preferences about what exactly that is.

Alternately Foggy just won't shut up about how much it hurt to find out that his best friend lied to hom since they met.