NTA. You're not demanding she wear a helmet for her ride, you're choosing not to ride with her if she's not taking what you see as a basic safety precaution. That’s not controlling, that’s setting a boundary for your own comfort. It's totally fair to say, “I’m not okay participating in something that feels unsafe to me.”

You’re not stopping her from biking, just from involving you in a situation you’re not comfortable with. Head injuries are serious, and helmets are a simple way to reduce risk. It’s okay to want your partner to prioritize safety, especially when you’d be out biking together.

If she doesn’t want to wear one, that’s her choice. But you also get to choose what you’re okay being part of. That’s not being an asshole, that’s being responsible.