I'd think that a standard Garand sling would work if your friend has standard 1.25" swivels on it.
Two other things that he should know, though:
1 - make sure that he's got at least 4 10 round mags - I know the shoot boss (I see him every time I brush my teeth or shave), and he WILL be doing a fair amount of rapid-fire AQTs.
(This is a four week warning, and I urge him to take it seriously - otherwise we'll be having to find someone to load his two mags for him as he's shooting)
2 - make sure that his sights or scope are LocTited DOWN. I watched a guy blow a Rifleman score and we couldn't figure out why... until we realized that the scope on his GSG had worked it's way loose. Heck, we had two scopes work their way loose this weekend! One lady would have cleaned the Redcoat target, I think, except for her scope mount working loose.
One recommendation would be for him to go to the range some weekend and blow about 200+ rounds through it in a day - see if there are any weak spots to the rifle. I see it EVERY DAMN SHOOT - someone brings a rifle that just got out of the shrinkwrap and we then have to spend half the morning troubleshooting the damn thing. He gets to waste his time - AND money - doing what he should have done before, and NOT learning.
I, and all my fellow instructors, want to be TEACHING, not troubleshooting. We have a very important message that we want to communicate, and we can't do it if a shooter is sitting there worrying about his equipment.
I know this sounds like a rant, and perhaps it is - but I believe that we can best serve our cause if we're teaching and not acting as gunsmiths.
So, Stratocaster... please invite your friend to go shooting, and wring that puppy out. Find any problems BEFORE your friend comes to the Appleseed Shoot.
Please? I guarantee he'll have a much better time if he has a reliable rifle!