Thank you for your query at DoctorSpring.com
She is already menopausal, so the endometriosis would be in remission now, as it is a hormone dependent condition.
I think it is totally okay to take aspirin if you find that this relieves her leg cramps.
Many people do take aspirin on a daily basis to lower their risk of cardiac events / strokes etc.
Further 100 mg is quite a low dosage and not associated with any adverse effects.
In my opinion, I think she can go ahead with the aspirin on a regular basis.
All the best
Please feel free to discuss further.