non-expert idea: show up at their place of residence often, ALONE or with 1 friend. At first, just knock and ask. Do this as often as every week or more often. Next month, show up with some more friends, who DO NOT do anything except they are just present and visible.

You'll either get paid partially soon, or paid in full. Or if you are unlucky, he will simply disappear if he has no attachments like a wife and children.

I've never done it (as I don't loan too often) but I'm sure if I show up with some friends and we are all in military uniform ... I'll get results.

If he's a friend, eh, just show up alone as long as you can stand it. You don't want to scare him into hiding.

I also know of some friends, who moved from one city to another, changed their last name (unofficially) and have not been bothered by their loan sharks for several years. It's none of my business so I don't say anything. But for sure I'm not lending them any of my own money.