in your sister in-law's case, if her friend was the person who set the terms, "friends with benefits"...he's already got someone steady, while she's his benefit. she'got nothing to gain...except the minimal time he spares online (when he's bored) and if both are lucky the occasional sex (phuck buddy), if there is any at all.
if one really wants to be friends with benefits, consider being the whore (goes for men, too) because you should never have just one! ha!