I don't think the donors can undonate. Once they make a donation the money sits "somewhere" (presumably a bank account or similar), but the claimant can withdraw it online it at any time. He can either withdraw it in dribs and drabs or wait until he reaches his target figure and then withdraw it all at once.
In this case the donations have already gone up by another £1,000 since I posted the original question (3 days ago). He is now only about £3,000 away from his target. We have suggested that he pay for his medical treatment asap so that he has spent the majoirty of the money. It just seems a shame that money raised for such a worthy purpose has to be counted and therefore lose him his HB.