Can my mother tandem skydive with fibromyalgia and rheumatoid arthritis?

Probably: As long as she does not have unusual bone fragility or joint movement limitation and with usual safety precautions.

Yes: However, i would not advise it unless the RA and fm were under very good control.

See your doctor: Your mother needs to check with her rheumatologist first. If she has rheumatoid involvement of cervical spine c1/c2, she does not need to jumping out of an airplane or else she could risk serious spinal injury.

Probably yes: Fibromyalgia is not a problem. If her upper cervical spine is not involved by her disease, and she does not have severe osteoporosis, she should be fine. Her rheumatologist will know. Quadriplegics have made successful tandem skydives. Expect an impact about equal to jumping off the tail gate of a truck, or less.