Not alot. He apparently worked at Barbary Coast in Houston and that is about the only fact. Everything else about him turns into everything else in this case - mystery and rumour.
DA doesn't remember if she met him 72, 73, 74, which is rubbish as you would know if you knew someone years or months, and would know if you were in contact with him while in ither relationships etc.
He supposedly called and invited himself for Christmas and she forgot he was coming till he arrived. Again rubbish. How did he know to call her at Minot if they had just casually met sometime, after all she hadn't been there long herself. Visiting over Christmas is different to visiting a random weekend, and if he was due to spend Christmas with her you would think some sort of arrangement would have been made to include him.
He got off work in Houston and drove all night arriving 3 hours later. First that was some fast driving, and second why not sleep a couple hours and arrive later in the day? Also notice DA doesn't remember meeting him, forgot he was coming, but knows exactly when he got off work and when he arrived.
Supposedly he just turned around and left immediately. Unlikely! I mean the guy has worked a shift, drove through the night, and doesn't even stay for a drink or to use the toilet? Which must be the case if TT is to be believed when he says he doesn't remember SM arriving at all.
My feeling is that either SM was mentioned because there was activity, new vehicles, outside Minot that night, and this accounted for it, or SM did visit as claimed but hot tailed it out of town when he realised something bad had gone on and wanted no part of it. I assume SM was spoken to at some point by LE but that would depend on how soon DA actually mentioned him and if he were traceable/ alive by then. He doesn't seem to have been in DA's life after that morning and no one else connected to the case seems to know him.
Edited to add that SM seems to have been a legitimate well established dj, likely older than DA as he was working at Barbary Coast in 1971. I found mention of him being a kind of mentor to a female DJ karen cook back in 1971 in a few places talking about her musical background View attachment 411237Karen Mixon Cook - Wikipedia