"To do someone¨ can mean to have casual sex. From the dialog, it appears that Frannie did not invite Monica to the wedding because Monica had had a sexual relationship with Frannie's fiancée. [former lovers are not normally welcome at a wedding ....] Notes - The humor in this dialog depends on a double meaning of ¨know¨ somebody and the shift between ¨what you did¨ and ¨who you did.¨ Frannie: I believe you ¨know¨ my husband. [intended meaning - you know him too well because you have had sex with him in the past] Ross: So it's really a question of "who" could you possibly have done. [intended meaning - Frannie is upset because of who you´ve had sex with.]

In the context “done” or “do” means to have sex with someone. I can’t watch that clip right now to confirm why he said that, but I’ve seen that episode.