He might have thought she was attracted to him. Anna Nicole Smith claimed to have been attracted to her 90-something husband. There have been enough historical examples where power or intellect apparently attracts. Or maybe he thought she was attracted to Epstein's money and wanted to be on his good side by "entertaining" his friends. Or maybe he was fully aware of the situation.

We cannot know what he thought, and we cannot even ask since he is dead, and we did not even know what actually happened after she "offered" herself (unwillingly and coerced by Epstein, of course).