Foreigners in the British military?

[Vortex what...ouch!]

I have a friend whose mother has British citizenship but he is Dutch and has been told he cannot claim a British passport. He is interested in joining the British military, but is not yet sure which branch, although he does want to fly.

Anyway the question is can a Dutch person join the Brit military? Anybody have a definitive answer of know where he can get it?

Thanks.