Mental healthcare for veterans is clearly grossly inadequate. Lobby your representatives for better support and keep lobbying until something changes.
If you're concerned about the mental wellbeing of someone you know, act early. Don't let your doubts stop you from acting, don't fall victim to wishful thinking. Talk to them and listen to what they have to say. Keep in touch, even if they're becoming more distant. Encourage them to seek treatment. If you're concerned for their safety, call an ambulance. Find out what your state laws are on involuntary commitment, just in case.
I'd highly recommend taking a course in mental health first aid. These courses are widely available and usually free. In just a few hours, you can gain the skills and confidence to help someone in a crisis.