Let's not blur the distinction between legal and illegal immigration.
I'm presuming that your father-in-law was a legal immigrant when he joined the Navy. Many legal immigrants have taken this route to citizenship and I have no problem with that whatsoever. I think that anyone who's willing to serve in the military should get a bump ahead of those who aren't willing to.
OTOH, I don't think that the opportunity to join the US military should be made available to illegal immigrants.