They are both wrong. You are normally guilty
of a crime but that doesn't work in your sentence.
Guilty about is possible in some contexts:
She felt guilty about that error in the statistics. (= She was at least partly responsible for the mistake.)
I can't think of a case where guilty for would work. I would say: I felt guilty because I had spied on her, or I might use a causal clause equivalent: I felt guilty having spied on her.
CB