Paying for a service and doing it yourself. 
Strictly speaking, he paid for the massage. If he'd given her a tip then that would indeed be madness. I've seen a few reviews of this place but no sniff of a polite squeeze of an buttock or a tit. It's not for me, but I can just about understand a punter who enjoys that and maybe, with multiple visits, he can lure her over to the dark side.
@saunaboy maybe leave an extra £20 on the side where she can see it next time to see what happens.