Mark let Bryon borrow what for his date with Cathy in That Was Then, This Is Now?
Mark lets Bryon borrow a shirt for his date with Cathy. Actually, he doesn't exactly let him "borrow" the shirt in the traditional sense. As Mark is leaving to go out with his friends, he casually calls back over his shoulder to Bryon, who is busy shaving, saying, "I found this shirt out in the street, and it's lying on the bed if you want to see it".
Bryon goes over to the room he shares with Mark and finds a shirt that just happens to be "just (his) size and dark blue, which happens to be a good color for (him)". Bryon wonders for a minute if Mark "had bought it or stolen it - they were the same thing to Mark - but (he) decided to forget about it", rationalizing that "it's the thought that counts".
Bryon had been "kind of bothered" about what to wear on his date. He and Cathy are going to go to "a casual dance", so it really isn't that important what Bryon wears, but he is "bugged" anyway because he wants to make a good impression. Mark, in contrast to Bryon, never pays much attention to what he wears, but somehow always manages to be "dressed right for the occasion". Mark, knowing how important Bryon's first date with Cathy is to him, senses that he will want to look nice. Out of love for his friend, but not wanting to make a big deal about it, he casually arranges for Bryon to have a shirt that will look good on him and be right for the occasion (Chapter 3).