I would not tell her until I absolutely HAD to tell her.
This is definitely Not the time to tell her...our son was in the Navy and I know the anxiety you feel at first - but then, you realize, you are not only proud of them for taking a stand - but you realize they need your support more than they ever have in their life!!
When she learns of that decision is not the time to spazz out like Cindy S. Cindy S. is a sorry excuse of a mother!