If it wasn't possible then not only would you have to pay a second time, but your mate would have to pay customs and import duty in the US when you send it to him. I know in the past when people have sent boards from outside of the EU for people to repair you can get them marked up in such a way that there's no duty for the repairer to pay. You wouldn't repair anything from abroad otherwise!
It's perfectly possible that you may have to provide evidence that it's going back for repair, etc., and I don't know all the details as I've personally never done it, but again, HMRC will be able to tell you.
It may be that you have to claim a refund on what you paid the first time round, and then pay it again later?