I think the best idea is to talk to your DM. I don't think us folks here on ENWorld don't know enough about your characters and players to really make a good call.
That being said, if I were in your shoes I would approach the DM and suggest that, at his discretion, that your cleric's spells/boons either would either not work or have the opposite effect when cast on the wizard. That way the DM can use it as a tool to make the campaign more interesting without risking a TPK/party split.
How awesome would be if *that* was how your character learns about the true nature of the helm? Imagine that the wizard drops, your cleric rushes to his aid to heal him. But instead the wizard shrieks in pain as a massive burn blossoms on his skin wherever the helm touches. Your Cleric would be surprised, the rest of the party would be shocked, and the Wizard would seriously need to weigh the costs/benefits of using the helm.
All that being said, I don't think the helm would require your cleric to leave the party. As long as the campaign is about fighting the greater evil, you can always find some justification to work together. I don't think many gods expect their clerics to only adventure with fellow followers.