Hes a young single guy,if his mate is of a similar ilk whats the big deal
That is a little like asking what's the big deal about punting. Of course it's not a big deal but many would beg to differ. It wouldn't be an issue now but if a mate were to grass him up in the future, there could be fallout even if it's very unjustified. To be fair, if it does end up having an effect on future relationships then they're probably not the best ones to be in, i.e. a girl shouldn't give a fuck what the guy did before they got together but it does happen. It's also hardly great to have a reputation as the guy who sees prostitutes, especially in some lines of work or when you have fairly small social circles. While a friend is unlikely to "grass" because they're implicated too, they may not think of the future consequences for themselves, for example they may be blinded by a desire to "get back" at yellowledbetter after a falling out or a fit of jealously.
Also, what of the mates who get his offer but turn him down (not that unlikely as I presume she's not an absolute stunner and it's a fair bit of cash). That's ammo that could be used in the future.
I am aware this is all pretty paranoid thinking and I don't fully subscribe to it but I would be extremely careful about who I approached with this "offer".