No offence but when the clamping company seem confused as to how you have just handed them back their clamp without having it officially unlocked, they are going to look for signs of damage, and most likely see where and what your friend did to it. Clamping companies do not take lightly to having their clamps "removed", if it involves any other means than a big wad of cash in their dirty hands.

If you have really welded the thing back up as good as you say you have, and there is no visual differences (what so ever?) they might just be incredibly baffled and let it slip. But if they come to a conclusion that is hasbeen tampered with your friend will be lucky if he gets away with just paying the initial £100 fine. If not... the price for a new clamp 2