In "That Was Then, This Is Now," does M&M stay friends with Bryon and Mark at the end of the book?

Although the book doesn't tell us straight out, I would say it is highly unlikely that M&M stays friends with Mark, who is in jail, bitter, and seemingly headed for destruction.  I think it is possible, but also not likely, that M&M will stay friends with Bryon either.

Bryon does run into M&M at the drugstore near the end of the book.  M&M is cordial but confused; he at first barely remembers that Bryon once dated his sister Cathy.  M&M, once happy and carefree, has been forever changed by his devastating drug trip.  He lives knowing that his body has most likely been irrevocably damaged, and that the flashbacks can return at any time.  Bryon remembers him as having been "such a bright, sweet kid".  He looks perpetually frightened now, and "his old expression of complete trust and intent interest (is) gone entirely".  It seems doubtful that he will be able to maintain his friendship with Bryon or anyone else unless he makes considerable progress in his recovery.

Bryon also has been changed by his experiences.  At the end of the book he is numb, "too mixed up to really care" much about anything.  It appears that he, too, will have to reconcile many issues in his life before he will be able to sustain any lasting friendships (Chapter 11).