Get an MMO mouse, they have enough buttons for all of your needs.
Reasonably you need 2 primary clicks and another 10 iirc .....
4 for abilities (default: QWER)
1 for self cast (default: alt)
1 for minion control, think Morde, LeBlanc (default: ctrl)
2 for spells (default: DF)
2 for quick-pings (forgot the defaults for this.)
So all in all 12 buttons, obviously it's a lot to mess around with on one hand, but I think it'd be his best shot. I've seen MMO gaming mice with 20 some buttons. And hey, Cyber Monday's coming up! try waiting till then and you'll probably save a decent amount.
Check out newegg.com. This exact link you can mess around with number of buttons to the left of the page. Good luck and I wish you and your bud the best!
P.S. The mice I linked are right-handed, but I believe there are left-handed mice out there. Just keep with the MMO-style gaming mice, they'll be the ones with enough buttons on the interface.