My eldest DS has just gone through the years induction programme, before being accepted as an officer cadet. He is 19 yo, and will be going to Uni in September for 3 yrs before going for the officer training. He has already been told that he will be posted to Afgan or Iraq.
We sat and had a long, long conversation about what he wanted to do, why, my worries, etc etc.
He knows that my position is that I would really rather he didn't do it, (I would rather stick pins in my eyes than him do this) but I must also understand that he is an adult, and is entitled to make his own decisions, and if my DP and I have done as good a job as we think we have, he is capable of making this decision.
I dont have to like it, but I do have to respect it. 
YANBU to expect that the father and son tell the mother, but IMO she is BU to not be supportive if the family have thought through, and discussed, all the implications, even the worst ones.