When discharging a patient from rehab who has used oxygen for the last 30 days, what should the discharge include?

You've already gotten good advice.  I can only emphasize that, if the oxygen saturation rate (SAT rate) was really 79, that's way too low to be considered acceptable, dangerously low.

It is possible she was able to breathe on her own, but if it was at a 79 SAT level, that's NOT acceptable.   Someone screwed up.

Is she still in the facility or at home?  If the former, have an immediate conversation with the DON and raise the issue of the low SAT rate.  90 used to be considered a sort of point of departure; anything lower and oxygen was justified.   But she should also be tested while stationary and while walking and moving around.

I wouldn't think about weaning until a pulmonologist has been consulted; it's not something for a family to initiate w/o medical support, including follow-up home care after discharge from the facility.

Guidelines would generally be what is the best for the patient.  If a night test confirms she needs oxygen overnight, the pulmonologist would advise continuation of that.   Home care nurses post discharge can take SAT rates on visiting and if they drop, the pulmonologist would be consulted to authorize and make arrangements for the oxygen supplier to deliver equipment (if necessary) to make oxygen use 24/7.

Didn’t you say GM was on oxygen at night prior to her recent episode? Did someone take her oxygen away while at rehab or did I misunderstand?
Was there any meeting with DC planning prior to her discharge home (via phone call if not In person)? I am thinking someone at rehab knew from GM’s past history that she was on oxygen at night prior to her admit there or it wouldn’t have been ordered. Is the order for continuous Oxygen via nasal cannula or use oxygen as needed?
Patients are often required to see their PCP within 2 weeks of DC from a SNF. I would make that follow up appt and discuss or verify GM’s oxygen needs with her PCP. One must qualify for Medicare to pay for oxygen and it sounds like your GM has already been determined to need oxygen prior to her fall.
Lastly I would have questioned the lack of an order for oxygen at home when picking up GM. Did someone go over DC orders with the staff prior to her leaving?
At this point I would get her in to see her PCP to resolve the immediate issue and then get your concerns together and inform the DON at the rehab center of the omission to assure it doesn’t happen to anyone else.
With Covid and SNF staff already stretched unfortunately I can see lots of room for error these days where DC instructions aren’t reviewed thoroughly. Not that it’s an excuse but they will tell you Covid is to blame (everyone is).
Good luck and I hope GM feels better each day.

With that bad of a sat there is no likelihood that grandmother will ever be able to e without O2. You will need MD order for o2 at home. What dreadful discharge planning; actually life threatening in the opinion of this old retired RN. Does Grandmother have COPD? Other lung condition?

Yes you do wean them off of oxygen. They should have been doing that in the hospital with a series of tests.