just be straight, say you dont want to fall out over money as he's a mate, but if you dont get the money back ASAP then you'll disown him.

to be honest I'd disown him anyway after you get your cash back, as he's obviously not much of a "mate"

In my 29 years I've found people are only out for what they can get and screw everyone else, hence now I have nothing to do with anyone apart from close family.