I would consult with my Rav before making any moves. I am aware that "sha'alos macht treif", but I just would not be at peace with myself.
I personally would be nervous. What if the woman or her family suspects that the money was in the desk? What if they come after me or my family for it?
This whole story is very strange. Who "misplaces" 100k in cash? Sounds to me like the money was intentionally hidden there by someone, probably NOT the woman, but a relative dealing drugs.