I dont think ethics are a concern here. It's just another selection. No different really than when you notice someone starting to open gaps, thrash around on the bike, miss pulls, etc. and punch it to pop 'em because you no longer need them in your break for it to succeed.
The ethics question to me would be that you notice they're out of water and you've got that extra bottle, or something along those lines - do you give them a bottle or not? I say yes to that one, and have done it on several occaissions when I've taken an extra hand up.