I think the idea that he'll hurt himself with barbell weight training is silly and 50 is by no means too old. If anything, an older guy who ISN'T going to go nuts adding weight and pushing hard on a novice LP Is probably the least likely to injure himself.
If he's in truly woeful shape or just lacks confidence you may need some kind of progression for squats like leg press to goblet squat to barbell squat.
But I think the systemic loading and fast results of a novice LP is probably the best driver to continue training further (and I bet once he sees how much stronger he is than was before he started and how much stronger he is in comparison to his peers he'll be hooked). He'll probably just have a much shorter LP than a 20 year old guy and will need to transition to advanced programs (or maintenance mode if he doesn't really want to push it) sooner.