Tricky one this...as a bloke who is online a lot, I often get spammed by this particular website, who's usual mail contains photos of 'kinkykate from (insert your home town here) is online now and looking for fun'...a few clicks on this and you find yourself at their sign-up page.
So, out of curiousity that that buxom barmaid from the local or the quite type from the office might actually be on there, what does our intrepid hero do - signs up in the hope of seeing someone we know/recognise in a state of undress!
Myself and a friend discussed this and took the bait, registered, and found that indeed all the adult friends who wanted finding were either trogladites or hookers trading behind a thin veil of online dating respectability!
There may be more to it than that, but just don't discount the prospect that he might just have been curious and registered to see what was going on.
Of course, if you find he has his own profile and states all manner of weird and wonderful sexual predilections, you may need a face to face chat!
Hope that helps....