I can't see there's much you could do, the fault would lay with you for letting the domain drop, if a mate of a mate caught it, I can't see the problem with that, it's just another catch to them, I can't see how you would have any rights to the name now it's dropped. Having said that, I'm sure I've seen a couple of stories on here recently where Nominet gave a name back to the person who dropped it, and it would all depend on the specific circumstances that led to the accidental drop, for Nominet to become involved, and done on a case by case basis.
If the partners were decent, they might say to their friend, hold on, our partner owned that name and he accidentally dropped it, the mate might say, is it important to your and your partners business, or he might say, not my problem, it dropped and I caught it, I suppose it depends how close that mate and the partners are as to what might happen to the domain.