. Its not illegal to use one without a doctors prescription. Its not illegal to change pressures etc. Its not illegal to show someone how one works, ie turn it on set its pressures, its options etc. DMEs and Docs would love for it to be but it isnt.
Now with that out of the way.
She really should have at least an in home sleep study just to rule out central apnea. Short of that if someone watches her sleep a few nights all night they can tell if shes struggling to breath or if she just stops with no effort to breathe. Visual comformation of what the chest straps on a home study do.
If its an auto and she has obstructive OA and no other health problems. set the min at a pressure where she doesnt feel smothered and the auto will do the rest. Best to have Sleepy head or software and track her results tweak her pressures etc but lacking that the auto can help quite bit without out though not as much as with it software to track.
Untreated apena is worse than less than ideally treated apnea. If you want the best therapy once youve ruled out Central or complex apena or some health problem that prevents using Cpap youll wind up doing it yourself anyway.