MetaChat is an informal place for MeFites to touch base and post, discuss and
chatter about topics that may not belong on MetaFilter. Questions? Check the FAQ. Please note: This is important.
Not area-specific advice, but one thing that generally makes me feel safer is having people around who will notice if I don't come home. I assume he'll have roommates or there's a supervisor or someone else around like that? You may just mention to him that it makes sense for roommates to kind of keep an eye out for each other or for residents to check in every now and then with the supervisor/advisor folks. There's safety and reassurance in community.
My impression of the Tenderloin is that it's more down-and-out than anything else. This is a totally different situation of course, but, for what it's worth, my partner and I (we're both male) used to hold hands walking through the TL and never had a problem (can't say the same for the Mission).