Sounds to me like you loved riding the mare and jumped at the offer for a free horse in spite of her injury. Sometimes the best deals are made when owners don't have the time and patience to see their horse through the tough times and it becomes a case of "their loss and your gain."
If she is able to bear weight on it and the swelling has gone down something good is going on there. While it might not hurt to have an xray done sometimes a wait and see attitude isnt bad so long as the horse is not being put through undue pain or stress. If you can afford a vet do give a call and have him/her come take a look at her and give you some advice about the best way to see her through this.
Best of luck and do keep us posted.