Do you mean since=since the time ?
Or since= because - for the reason that ?
1- He has not been able to go out with his friends since (since the time) he broke his leg.
2- He has not been able to go out with his friends since (because - for the reason that) he has broken his leg.
You say that his leg has not been healed yet. You are right, but when we say since, and it means "since the time", then we should refer to a point in the past when something was done or happened. It's a point so we should use past simple after it. (sentence no.1)
You are told that you must use present perfect after since. But this one is not "since the time" since, it's "because" since.
Why? because he has broken his leg and it has not been healed yet.(sentence no.2)
Student or Learner