What about libelling him, then having him sue and you settling the suit?
Nice!!
I know from experience that winning 4 figure amounts does not trigger any tax investigation, (very infrequently), one does however keep a copy of the winning docket (but as most such wins are through accounts with bookies, there is a clearly defined audit trail here, so there's no problem), in addition, payments are normally made directly from the bookie to the bank account, so again the money is traceable.
In Booh's example, i think the following would transpire:
1. There is no guarantee that the revenue would ever know anything about the transaction, however it's likely that the bank's money laundering officer would not accept your explanation and thus alert CAB!! Your account would then be frozen! This is presuming we're talking about your 100k here.
2. CAB don't prosecute but alert the revenue to the transaction, the revenue will apply CAT most likely, but this should be challengeable if you can get as many witnesses as possible to sign sworn affidavits to the effect that the money was won in a genuine bet, not set up for the purposes of gifting 100k to your mate (or vice versa), this will of course be perjury, and the revenue may not accept the explanation anyway, they are notorious for being ________. If you can afford to take a case challenging them, you might win, but you might not. If however you decide to gift your mate 3K per year for the next 33 years, there's nothing they can do. In fact, following on from that, could you give 33 mates 3K each, then each of them could gift 3k to your other mate, which I think may not trigger CAT, although you would end up with the same scenario as previous, ie the revenue would apply anti-avoidance rules and tax your mate anyway.
Remember nothing in life is certain except death and ________.