Context
Recently, I had meeting with my bosses to discuss my performance. At the end of this meeting they asked if I could help them by "putting out feelers" for a possible replacement for someone who has left our company. I ended up emailing them the resume of one of my friends (after confirming this is a role he would be interested in) and fast-forward a bit and he has received + accepted a job offer. We will be doing slightly different jobs (soft. dev vs IT tech) but due to the nature of my workplace I will have to do some things that he would. In addition, as a more senior employee, I will be partly-responsible for training him and managing him.
Problem
There are lots of things that people believe can go wrong when working with a friend but I haven't seen exactly what those are. When working with my friend, what are these things that can go wrong and how can I avoid them?
My thoughts
I was unable to find a similar circumstance on stackexchange so far. The most relevant questions are to deal with having a friend do work for you. The main principle that I took away from these questions are: setting clear boundaries and expectations. I'm just not sure how I would do that though.
Edit for more info Per @dhein's comment. I mostly want to keep things separated in terms of friendship and work. Of course, I'd like to have the regular "water-cooler" conversations and amicable relationships you'd normally have with co-workers too. I'm not sure exactly what my friend expects but I feel that this is what he's been advised to act as well. I was more so looking for potential pit falls that I may fall into that causes me to cross a boundary