Whilst he shouldn't return too early to cause his injuries no time to heal, or be in pain. That behaviour/attitude is frankly appalling. Really poor work ethic, sure big corporations don't care probably, and you don't necessarily owe them anything, but that attitude is pretty poor. I hope it doesn't reflect what modern society and its behaviours in the workplace is like.
Thankfully managers generally are wise enough to such things and you only do yourself out of pocket in the long term. Forget promotions etc.
As said see what the specialist says. and then talk to manager. Depending what your job is. They maybe able to put you on more suitable jobs. No reason at all to have a long time off, with a broken leg.
As for doctoral note. Had no issue returning earlier, as most doctors note these days, have a line in saying something along the lines 4 weeks. Unless he/she feels ready. Basically giving you the option to decide when you feel ready.
After breaking sternum, went back to work after 5 weeks. But in an office duty, then after another ~5 weeks went back properly and anything I couldn't do, like lift objects, other people just had to chip in. If I waited until it was fully healed, would have been off work for ~18months. Best thing is after the specialist talk to your manager and see what all the options are, having lots of time off work sounds great, but your spending money, rather than earning. So its pretty much in front of the TV and that sucks after a few days. Management are usually very understanding, as a lot off bonuses have things like sick rate included. So its better to have you back at quarter capacity than on sick, when I went back it was office based and I could leave anytime I wanted, if I was in pain/tiered etc. Others might say there's nothing you can do and stay off, but at least you've talked to management and kept a decent working relationship with them, rather than just handing in one sick note after the other, with them not having a clue what's going on and so haven't rely got a clue how long they need to replace you for.