Well it sounds like one of two things, either your mate could have problems with his master cylinder Mordy, or he's got a stuck caliper piston.
Like on our old bikes, there's a port that allows the brake fluid to flow back once the lever is released. If this little port gets blocked with dirt, the fluid can't bleed back, so the fluid stays between the M/C piston and the brake caliper pistons (good ol' twin pot Brembo's, excellent brakes!) virtually locking the brakes on. He'll need a piece of fine wire and some patience, but he should be able to unblock the port without disassembly of the MC, although replacing the fluid and bleeding the whole system is recommended.
If it's stuck piston/s in the caliper, the best thing he can do is pull the caliper away from the disc and while the brake hose is till connected, pump the pistons right out, clean any corrosion out of the caliper and off the piston itself, (or replace them if they're too badly corroded) replace the seals, (use a little rubber grease when you install 'em) pistons, dust covers, re-install it on the bike, bleed the brakes with new brake fluid, and he'll be right to go! Cheers, Terry.
