Without knowing more about this person and your relationship to them I guess my very cautious way to approach it would be to mention in a vague kind of way how -some people- might find working for an arms-maker a troubling thing to do and given the controversy around such things they should perhaps give thought to whether they feel it's a good fit they'd feel comfortable with and good about. That it's nice to have a well-paying job but would it be something that would cause them any kind of stress or moral dilemma that would make the job harder or would it be something they truly are comfortable with.

I do think mentioning the pigeon-holing cuddlefish mentioned would be good too. How unfortunately once you get experience in a specific sector like this you tend to be guided into jobs like that in future and is that the kind of career track they're hoping for or were they hoping for something else in another industry and if so wouldn't it be better to take something that leads more clearly into that rather than this?