It certainly is possible that the sore is being caused by constant irritation from the broken filling. However, as Christine said, it is impossible for me or anyone to tell you what it is over the internet.
The only way to find out what the sore is, is to have a biopsy. It certainly has been there too long and as David said, anything that is there for over 2 weeks, should be biopsied.
If the tooth can't be fixed, the least that she should have done for the time being, is to get it smoothed so that if it is causing irritation, that will stop.
Jerry