Thanks for the vote of confidence - ha ha. Besides being a coach, I'm pretty knowledgeable about human movement dysfunction and painful syndromes associated with such. I also work in the fitness industry and deal with people who come in with a laundry list of ailments that we have to find ways to train around. That being said, here are a couple of other things to consider based on the responses to your questions...
Since she's got the big arch in the back, that's going to make her very front-side (anterior) weight bearing in order to properly position her center-of-gravity during her upright stance and posture. That will cause the calf muscles (gastroc/soleus - plantarflexors) to be very active all of the time. The achilles tendon is an extension of the ankle plantarflexors and I could see how this would lead to constant pulling on the heel bone (calcaneus) and intensify the chronic inflammation.
I would really suggest seeking out a great physical therapist to evaluate her and to work towards improving her core and pelvic control. Secondly, I'd highly suggest possibly finding a good manual therapist (massage, etc.) to do some work on her lower extremity to loosen up the musculature down there. I'm sure that the gastroc/soleus muscles are very "tone" and probably have trigger points, etc. Plus, I imagine the fascia could use some re-working as well. Fascia is not something that can be stretched; it's very mechanical in nature much like muscle. It needs to be worked out manually.
I'd also have the PT look at her pelvic alignment and how that may be related to things down the chain. It seems odd to me that she's got high arches. Most people in an anterior pelvic tilt tend to pronate (roll inwards) or have flat feet because the anterior tilt causes inward (internal) rotation at the femur and the tibia. Her high arch might actually be a compensation for her pelvic alignment. That being said, her compensations at the feet may be contributing to this issue, too.
If you'd like, feel free to PM me and let me know where you live. I might even be able to recommend a physical therapist or otherwise in your area or may know someone who might know someone in that area of the country.
Good Luck. Take care.