That’s a hard one. Most importantly, if you are going to bring it up, do it privately. Never, in a million years, bring it up with other people around. That’s a big no-no!
Moving on. Tread carefully, because many people feel very uncomfortable when people point out their scars. If you were to do it, and whether you should do it all depends on the situation at hand. It all depends on how well you know them, how comfortable they are around you, and just their personality in general.
No matter how you go about this, there is a good chance it’s going to become uncomfortable for the both of you, but especially the person. Honestly, my best advice is to do it gently, and do it in private. If their immediate response is to shy away from you, do not push. That will make them feel trapped. If they do shy away from you, reassure them that you care, and that if they ever do want to open up, then you’re there for them. After that, drop it. Don’t treat them differently than you did beforehand.
Lastly, about the hugging, that again all depends on the person. Some people don’t want the hug, but some people crave the physical comfort. Ask them beforehand if it’s okay to hug them.
Furthermore, if you’re worried about their immediate well being (ie. they seem to be endangering themselves) please, please contact professional help.
I apologize that this is rubbish advice, this really is a hard question to tackle, and each case is different. Some people want to hide it and keep it private, some people want the help/comfort. It all depends on the person at hand.