Sportstem,
What you ask is very sports first aid specific. If you see a badly fractured bone with dislocation, displacement or angulation or an open fracture (skin wounded, not intact), where nerves and blood vessels are at high risk, then immediate treatment is necessary. This might involve manual bone reduction to avoid nerve and blood vessels failure but an MD is the only responsible, not a physio (unless trained). This means that an MD must be present at the field. If not, wait for the ambulance, you shouldn’t try any reduction. Leaving the shoe or not on the athlete's foot is to be decided at that time. In general, when someone is not sure of the consequences, then no cloth/shoes removal should be performed in order to prevent further damage. Apply RICE if you can, wait for help and encourage the athlete to keep him/her conscious.
The rule of RICE (Rest, Ice, Compression, Elevation) applies in all such injuries but only when fundamental body functions have been secured first. Always remember the rule of ABC in first aid: Airways, Breathing and Circulation. These are top priorities in case of an emergency. All other health problems including a fracture must follow. So, in case of a fracture, always check the athlete’s consciousness (a leg fracture could be accompanied by a neck injury as well and the latter should be treated first). If he/she is conscious, then check for his/her breathing. If breathing is ok, check the pulse and any obvious bleedings. Treat any bleedings and then check for any injuries like fractures.
I hope I helped some how. If you are in the medical/paramedical stuff, I suggest you take some first aid lessons every 2-3 years. That's what I try to do.
ilias