Lemme take #3, the last one, first...and I hate to answer a question with a couple of questions, but it does beg for it: What broke up the friendship? Do you think, or does he give any indication, that it's possible to repair the friendship? Does he seem lonely since that loss?
#2 and #1: Start by trying to be a friend...be observant of attitudes, likes and dislikes...above all: Pay attention. You can't tell if someone's gay if they don't display the fact, sometimes even if they do act 'gay'--whatever that is--so you have to become a 'fly on the wall' and see what's going on. Some will disagree, but if you met me, for example, you'd never know...really....jeans, t-shirt, beard whiskers, a little extra wieght...it just doesn't scream, "I'm GAY!" at anyone. Once you've got an idea of these people's attitudes on LGBT issues, then you can decide your next choice of action.