I know several people who were at that island and have discussed this event; one even told me that he remembered it because Marvin came over to him and said "this woman just offered to have sex with me." Also Gloria, his wife, was there, though I haven't asked her about it (and wouldn't). This seems believable to me.

OTOH I did read Giuffre's deposition and she says not just that she was told by Epstein to proposition various people but that it happened. I find that very hard to believe having known him so long, but she made that statement under oath. Also I'm not sure Marvin was famous enough to be worth making up a story about (as opposed to, say, a famous heir to a throne).