If the design does well, I don't think a beer would make it even.

Seriously OP if you want to do that, just be aware that hiring friends isn't always the best idea, oocdiddy's right about the payment tracking. If things get messy it could ruin a friendship.

Are you thinkng of giving your friend 100% of the profits from the shirt? Because if so, you'll be doing admin work on his behalf and he'll get everything. But if you ask for a percentage of the profits your friend could call you greedy, and that'd strain your relationship as well.

Hiring strangers is nice because you can always just do one-off payments, but with friends it's kind of hard because they know you personally and might ask how the shirt design is doing. If it turns out to be doing really well, they might get resentful and ask for a bigger cut. If it turns out to be doing not so well, then you've just paid someone for a crap design, and you might start to feel ripped off yourself.

As long as you have your friend's permission to use the design for commercial purposes, pretty sure it's not against any ToS. But it's never really a good idea to hire friends.