I think she should have another PET scan to clarify the situation. This way any change can be seen since the last scan & also the degree of any spread.
What does your Mother want to do at this point- treatment wise? She is obviously a great fighter but it's so hard to make these big decisions when one is in so much pain. Perhaps you could see if there is anything to help control her pain more?
Perhaps you should all meet together- your mother, her oncologist & yourself. The Oncologist could discuss the possible options with you both & you could take it from there.
I wish your mother & you all the best
Please let us know what you decide!