I see this as a post better suited to the forum, “Newer Divers and Overcoming _______”.
I know of no physical explanation for any given specific depth over and beyond any other depth that would create or increase the symptomology of what you describe, if it’s happening at 75’, it will happen at 10’. Now, you’ve spoken with a Physician, so they’ve ruled out medical causes such as a weakened Cardiac Sphincter.
If a diver is swallowing air, yes that can complicate buoyancy to the very small extent represented by the volume of air that a human stomach can hold. At the high end (without breakfast and in a male) that amounts to 1 liter.
That liter is not much air to compensate for by adding a bit more lead weight, pre-dive.
To look for a dive site based on the premise that it is easier to manage hyper buoyant objects at shallower depths is faulty on its face. It becomes easier to manage buoyancy once past the shallows, that magic number is different for every body type (wetsuit thickness, body fat and whatever amt of stomach gas retained), but many people stumble past this into “stasis” at 18fsw.
If stress is causing her to swallow air, I do not see any shore dive entry as suggested above in Bonaire to be a stress reducer of any fashion. They are quite simple by any standard, but they do have unique challenges can be quite stressful just to start, and adding stressors of the actual dive beginning inherent to that entry. Lots of new stuff. Any less stressful than sliding off a step of a boat, all positively buoyant, floating till you’re relaxed enough to slowly deflate and.....no- no Bonaire shore dive is easier than that simple entry.
So, in my experience, a shallow dive profile will complicate the inherent inability of a newb to present a flat dive profile, and will always result in a sawtooth pattern, usually with the diver making ongoing uncontrolled ascents followed by repeated fights downward.
As to having her stomach contents appearing in her esophagus and mouth? That tells me a few more things. Number one is: her stomach was not empty, so let’s cut that theoretical max buoyancy of 1 Liter down to a more realistic 1/2 Liter. The other huge factor is: What’s her u/w attitude? Gravity dictates that she is “head down”, otherwise she wouldn’t be burping her biscuits. She is probably head down because of over-buoyancy. Easy answer: stop doing that.
The lifetime, overall, simplest fix for your intended lifetime dive buddy? Get her a one-on-one instructor who is a real-deal expert on buoyancy. Not all of them are, you haven’t found one, yet.
Also find an instructor of an empathic nature. Your dive buddy needs to find a path to relax, calm down, and decide if this is what she wants. If you can combine this with. “Naturalist” Course, something to distract her from that “diving thing”, she just might forget about burping.
Fix the problem, don’t avoid the symptom.
