I've actually managed a friend whom was hired to fill a role under me. (so unlike you where you were promoted past him, I was already there when he came on board)
One thing I made clear with my friend was when at work I am a professional, I take my work seriously, and I'll do what needs doing to accomplish my work. When we're out to lunch or off the clock we can be like we always have been, but on the clock I expect him to work as hard under me as he would anyone else.
Separate Work and Home
One thing my friend and I used to do is have sort of a way of visibly demonstrating if we were in work or home mode. With the nature of the job sometimes when I was "on the job" and my friend was not he needed to be able to tell incase I had clients around, etc.
For us, if my shirt was tucked in I was in work mode and needed to be treated as the boss, when I untucked it we were back to the everyday.
Don't let work poison home
While I took the professional side very seriously and at work we kept it as professional as possible. (some joking happened, but it was pretty mild) One thing we decided in advance was we set only one "not at work" time to talk about work, and that was reserved for venting. Otherwise work was a forbidden subject. When one of us started into the subject we'd say "Rule 42" (we didn't have 42 rules, it was just a nod to Hitch hiker's Guide to the Galaxy)
Since I'm a straight forward but fair boss things went pretty well. we kept work and home separate. There was no noticeable effect to our friendship, and I got a lot more work done because I had someone I could count on working for me.
Neither of us were married when it started, but he was married to one of our colleagues before it was over, between the two of them as well as us collectively same rules. At work we were professionals, when we get off work, work becomes a forbidden subject.
Would I do it again? Absolutely! I've only had two people I've had work under me who worked even remotely as hard as he did, and that "time to vent" was always a great stress relief from the office.