Seconding what everyone said about the driver wearing it during a quick getaway. He can close his eyes if he needs to; she can't. Even if no debris or bugs hit, the wind from going freeway speeds is fierce -- think about what a windshield does for you in a car!
I'm wondering how he rides her bike when he's 10 inches taller than her. He must have his knees around his chin.....
Not necessarily! I've ridden the same bike as people a foot taller than me with no modifications, and no feeling that I or they needed them. It depends how it's set up, obviously, but it's totally believable. It's not like a bicycle where the fact that you have to pedal means your legs must reach through a whole cycle (the seat height makes a much larger difference on a bicycle).
And really experienced riders sometimes do enjoy riding bigger, more powerful motorcycles that newer riders would feel were too big (like, they can't flat-foot 'em).
Helmets come in sizes. I'm not going to believe a helmet fitting both of them. The size difference is too great.
Head size does not have to have any correlation with height. My mom is a titchy little thing and smaller than I am, and she wears a helmet that is two sizes bigger. It's a little coincidental that both your characters would both have the same helmet size -- just cuz, coincidence* -- but it's not unbelievable in the sense that her head couldn't be big enough.
* Though at at least in the U.S. helmet sizes aren't done by inches but are S/M/L (XS/XL etc) so it's not THAT much of a coincidence, and I could wear my size or the next size up and be okay. If you want to lampshade it you could say something about her jamming his helmet onto her head and a fleeting thought about how he's got a smaller head than she does despite being a foot taller.
eta: As for the intimidating approach, I'm tempted to say she should still drive, because she can come in super fast and skid to a stop or something and look badaaasssss.....but if it's specifically about making HIM look macho, and it's just around the corner, I'd say she should wear the helmet. He'll look more "macho" to thugs (or whomever) if he doesn't have a helmet but his little hot girlfriend on the back does.....er, at least, I would think. Not a thug myself.