Slock.it is trying to downplay this attack:
If he does not downvote it he will lose all of his money. So let’s assume he does that. The minute the attacker downvotes the proposal his tokens will be blocked and he won’t be able to execute any subsequent splitDAO().
If he's smart, he won't be blocked for long. He doesn't have to vote the first day, he can vote the last day and is only blocked for couple of hours. Not enough time to split by yourself.
The attacker has no guarantee that he will always get the transaction in the right block. The first time he fails he loses his money. The likelihood is that he will fail. Given that the attacker has to win every time and the victim has infinite attempts. It becomes a certainty that he will escape.
I don't think he can lose his money. If you successfully split without him following, he's not stuck; he can get money out with his own split just fine.
"Split at the last minute" is the only option left. You only have one try though, not infinite. That's because if you split at the last minute, 33% of your (and his) funds will go into extraBalance of the new dao and those are impossible to retrieve if he's blocking your proposals. You can only retrieve those funds if you both cooperate.
There is a better way though. You'd have to split not last minute, but right before 5% price increase. This way, if he's not fast enough, he'd lose 5% to the extraBalance. But that goes for you too. If transaction is not confirmed in time, you lose 5% too. You'd be walking a fine line. Fast enough to be confirmed (sometimes confirmation takes a minute or more) and late enough to not being followed.