Enlisting in the military is a big deal. My closest friends would say the same to me if I enlisted without asking for their thoughts, first.
Sounds like Sean thought they're closer friends than they are. They're probably going to mourn that realization for awhile.
Edit: Sometimes we have friends that we decide to be there for, in spite of a lot of stupid and drama. It's expensive, and I have sometimes regretted it. But more times it's turned out well for me. I would give them space, but remind David to take the time he needs.