Cheapest is not always the best, this decision is based on what you want to do.
It sounds like you may also want to look into getting a Health Coaching Certification! It's awesome that you lost 110 pounds!
How about doing ACE personal training and ACE health coaching.
Also, Join IDEA-it will ...
I would do research on ACE, ACSM and NASM
Take a look at what they offer and see which one is the best suited for you.
I must say that ACE is great because they have tons of resources and being a member of IDEA gives you support and unless resources
ACSM is the gold standard because this is where all research is done, however, ACE is great because of the access you will have in the industry. IDEA makes it very easy to stay current and keep up to date on events, research, CEC's etc.
Sometimes pain is related to certain medications. I would have her check in with her doc for side effects.
Stretching is ok but it may not alleviate her pain.
Before she proceeds I would suggest she does a bit more research into the cause of her pain.