I agree. Besides, who’s to say he’s wrong? Maybe that’s the way he’ll find happiness and be able to be himself.
SnoopyFan nailed it. The guy got a tattoo and wants to ride a motorcycle. Oh, the horror! Why, surely next he’ll start peddling drugs to the kiddies at the playground.
Men are from Mars.
Women are from Venus.
Brothers are from Uranus.
My opinions:
It’s important to be kind to your girlfriend and help make her feel better, at least by listening to her.
It would also be good to talk to the brother and encourage safe behavior.  Lots of people get hurt on motorcycles.  The whole package of head shaving, tattooing, and piercing often makes a statement of rebelliousness and edginess - it’s a superficial thing of course (literally modifications of the surface of one’s body) but symbolism is symbolism.  These activities should make your girlfriend nervous - they well may be intended to.  And most of us weigh the reactions we get from everybody around us as part of our process of choosing what to do with ourselves.