A lot depends on the grade of separation, when I did my left shoulder it was diagnosed as grade 2, and basically I'd describe it as being weak for about 2-3 weeks, didn't need any painkillers or miss any hockey games(it was the playoffs
) and was at 100% within about 6 weeks.
Then I did my right shoulder and it was diagnosed as a grade 3 or borderline grade 5, arm was in a sling for 2 weeks, taking painkillers as fast as they could perscribe them. Was sent for xrays/ultrasound and was given the option of either rest or surgery, choose rest/physio. Stayed off skates for 2 months and took another 2 to get back to about 80%. It's 18 months down the line now and I still don't have the same mobility or strength, probably only 90%, and have a pretty big bump on my shoulder. My job involves carrying some heavy things on my shoulder quite often and occasionally if I don't position it right the pain is exruciating.
Regardless of whether he has surgery or not, it's an injury more likely to happen after you've already done it once. Still not sure if I'd choose surgery if I turned back the clock though.