Wil: It's against the rules and not being forced to move using both dice values is a known bug. Until implemented properly it's either a question for your own conscience and sportingmindedness (to play by the rule), or it's a decision that both players should agree to (to take advantage of the bug). both of those are fair ways to deal with it. I know it's a tourney loser but I'd make the correct move 24/20/15 (but then, I was on the wrong side of this same situation and wasn't pleased that the guy took the illegal route without hesitation. He's just been banned for cheating - multiple nicks and rate fixing - but that's a different story, lol).
You could put the question to Sharon - ask her is she's willing to take the gamble. Your chance of winning would be about 1/40. ;-)
If you want to see what thoughts there were last time, stick bug into the search box on this page. It's the same rule but in hypergammon.