Eternal Doctoral Thesis
The tricky aspect of this question is that you don't seem to know for sure how much progress he is actually making.  Ten years is a long time to spend in graduate school, but not unheard of.  If he is slowly making progress and supporting him isn't too much of a burden on his family, then he might be better off finishing in eleven or twelve years total, rather than quitting after ten.  (Finishing may feel much better than quitting, even if he doesn't get an academic job at all.  There are also different sorts of academic jobs: he might benefit from the degree if he ends up in high school or community college teaching, where having a Ph.D. could mean something and they might not care if it was a long, slow process.)

If he is making no progress at all and will most likely never complete the degree, then doing something else with his life would be a better choice, but of course that's a very difficult conversation to have.  If this is the case, then his advisor should already be dealing with this issue, but it's possible that the advisor is avoiding the topic.

Another difficult case is if he could complete the degree, but nobody wants to support him for long enough, or if his plans for what he'll do with the degree are unrealistic.

My recommendation is that you shouldn't intervene unless you have an exceptionally close relationship (romantic partner, best friend, etc.).  Otherwise, it's really none of your business and I doubt you'll have a productive conversation.

If you do have a close enough relationship, then the first step is to have a honest conversation about how things are going, what his hopes are for the future, what's realistically possible, what the trade-offs are, what his fallback plan is, etc.  Without that, all you're doing is guessing.  Merely having this conversation could be a useful way for him to clarify his thoughts, so I'd start with that rather than focusing on making changes.  Discussing potential changes can wait until you have reached consensus on the situation he is facing.