First hand information here, not gong to say anything more specific than that (on this person's request), but just trying to help avoid the spread of inaccuracy.
Both divers were on single tank, open circuit. There was no deco obligation. The surviving diver was a very experienced cold water rebreather diver. I don’t know how many dives the diver who passed had, but she was apparently not a new diver.
Gas supply was not an issue. The victim had a free flowing regulator just prior to ascent after an uneventful dive, was given a working regulator, and rejected it in a panic spiral. Despite the surviving diver’s attempts to surface the victim, she was overweighted. The difficult (and correct) decision was made to avoid a dual fatality.
I was on this boat and towed Caroline from the diver who brought her up from the wreck to the swim platform/lift. The diver who brought her up was a CCR diver but not her buddy and was weighed down with gear. I jumped in with suit and fins (not even a mask) to expedite her recovery. Otherwise I was not involved in the CPR/resuscitation except to prep the AED. The AED never found a shockable rhythm, but CPR was continued even after she was transferred to the Coast Guard rib and then the paramedics at the dock. She was never stable enough to transport to the hospital.
@doctormike description is more or less accurate, minus a few details.
One thing missing is that Caroline was ascending rapidly at the time and the buddy vented her suit to try and stabilize her, the buddy now being light (from going up 15-20ft) and Caroline being vertical and vented - heavy almost for sure contributed to their midwater separation. There was apparently no affirmative "hold the strap" kind of connection between them.
The donating buddy (who was also in a single hp100) had 3 working regulators on them, long hose primary, a short hose secondary which was not attached under their chin, and a pony bottle on their right hand side (why right side I do not know). The buddy donated the pony bottle regulator to Caroline. We don't know exactly why the donated reg was rejected: 1) due to a panic spiral, 2) Caroline was too flustered to purge it and inhaled a slug of cold water and spit it all out 3) if the pony bottle was not fully turned on or not turned on fast enough, or 4) was actually turned off the whole time.
Caroline's cylinder had "250psi" on her gauge (again single steel 100) on the boat, but I don't know if/when that pressure was confirmed by another gauge. The police took her equipment directly off the boat.
The overarching take home messages from her death: 1) do your weight checks at the beginning and end of your first dives, take the time and energy to get this right, especially if you travel someplace new with new-to-you exposure protection 2) practice donating and receiving gas, 3) grab a stressed buddy and maintain positive contact like you were taught in OW 4) do not donate a regulator which isn't immediately working to a stressed buddy.