hey, Mark,
This is a really important question, & most of us practice as if it was all just hypothetical. If you dive enough, you will see many things that "almost never happen." OOA divers are not necessarily composed & mindful of protocol. Neither are they the #$%holes that some posters have implied. My OOA buddy had his mouth at about my waist level when he realized his plight, so it was natural for him to reach for my Octo (on my "golden triangle.") Having been in this situation recently, I have a few thoughts:
1) Own a good Octo. Either you or your buddy will be using it, & if it is too hard to breathe on, panic will threaten both divers' safety.
2) Hold fast to the BC strap or the other person's arm during practice; current will pull 2 people in opposite directions & potentially rip off mouthpieces, hoses, etc.
3) Too bad if your buddy is not keen on practicing this; her life is as precious as anyone's & may depend upon this procedure becoming SECOND NATURE, not just a concept. Just explain ahead of time before each trial what will happen so she doesn't freak in the pool.
4) In any situation unfamiliar to you or your buddy (deep dive, low viz, heavier current,, etc) expect that breathing may be heavier due to nervousness, extra safety stops, whatever. Just insist on ascending with extra air (pretend to be a wimp yourself if that is what it takes to convince the other that this is needed.)
5) there is no need to "rip" regs out of anyone's mouth or shove them in. This can break teeth (had a buddy in my rescue class that got real crazy with force in urgent moments.) These times require a calm & deliberation that puts the other at ease. Gently remove & hand off mouthpices. The vistim will accept them gladly; the donor will give them gladly if the signals are clear. PRACTICE "CALM."
Hope this prepares you for many sessions of practice. Every pool session is an opportunity to refresh the many safety drills we learned in OW. If there is not a pool that allows scuba gear, shallow dives in familiar places can start with a drill. The day you need it, you will be glad you did your homework!