All the precedents for Helms of Opposite Alignment I've seen haven't given any feedback about failed attempts to change alignment. Given that there's no description in the item of effects from failed attempts, your stretch of reasoning falls into fiat.
That said, put him into a deep, dreamless sleep (it's a first level spell, requires willing target) for the duration of the helming.
Or, to put it another way. You're a very good, kind loving person. You suddenly wake up. The last thing you remember is someone saying they're going to cure you. Before then, your memories are of being a horrific psychopath.
Do you hold a grudge?
Keep in mind, if you really don't want those memories, a decent bard can change them...
Also, here's how I expect the conversation to go:
"You fiend, you're imprisoning me! And changed my alignment!"
"Dude, chill out. You agreed. But if it bothers you that much, I'm tearing up the contract"
"Um... well... you still changed my alignment"
"You want I should change it back?"
"Well, no but..."
"So what's the problem?"