I don't really think it's an issue. So the horse got away. So what? One time shouldn't be a big deal for the farrier. He might be annoyed at losing money, but as long as it doesn't happen often, the farrier should get over it.
As for not getting your horse done, well, what did you want him to do? Spend two hours chasing your horse down? He has other clients to see. If your horse was where he should have been, the farrier would have fixed him up. Since he was frolicking in the field chasing your sister, the farrier had no obligation to fix him. He's a farrier, not a horse-catcher.
A $40 tip normally would be a bit overboard. I don't even give my farrier a tip. But since he was supposed to have gotten paid for shoeing your horse, and couldn't cause your horse got loose, I would probably have paid the guy anyway, for wasting his time. It's not his fault the horse got away.
And I don't think a missing shoe requires "immediate" attention. The horse can live with waiting an extra twenty minutes while the farrier works on the other horses.
Tell the farrier you're sorry the horse got loose, and that you couldn't be there. Ask if he wouldn't mind coming by to get that shoe on whenever he has time. If it's a big rush, just have one of the neighbor's farriers slap a shoe on if they're in the area.