The doctor has told my Dad not to drive anymore. My Mom still 'monitors' him and lets him drive. What can I do?

They can be sued for every penny they have if they cause an accident at this point.

Time to move into the role of parent, I'm afraid, and take the keys away. Disable the car if you must. Clearly your mother is not competent either.

You're absolutely correct that it sounds dangerous and absurd for your mom to say she's "monitoring" your father's driving.
Please tell her that a car is not the same as a jet. There aren't two sets of controls like there is in a cockpit so the captain can give control of the plane over to the first officer and vice versa. There's one set in a motor vehicle and control belongs to whoever is behind the wheel.
Your mother doesn't seem to understand that she's not the only person being put at risk by his driving. Every person on the road is also at risk, and she's going to "monitor" the situation right into a serious tragedy.
If dad refuses to stop driving and mom continues to be supportive of this dangerous situation, then you will have to act.
Go to his doctor and explain, contact the local police, and your state's department of motor vehicles and tell them about it too. A call to APS as well because this is a dangerous situation.
The other people on the thread make some good points. You may have to disable the car to prevent them from driving. Do what you have to do because you'll be preventing a tragedy.

How in the WORLD can you monitor someone else's driving? By screaming at them as the crash into another car? Grabbing the steering wheel? I can't see a case where 'monitoring' helps at all--in fact, can cause more damage!

I know almost no person who gave up driving because they felt they were no longer safe drivers.

Sometimes, and this is one of them, we have to stand up to our folks and be a little tough. For the safety of others, if not for our own.

Call his Dr. and request that he notify the DMV to have Dads license revolked. I agree, that Mom has some some decline here, too, if she thinks allowing him to drive is OK.

This is sadly not uncommon. Does your mom drive?

When my DH aunt had to stop driving, it caused a BIG problem for her SIL, same age but who had never driven, never had a license. Aunt would be annoyed that she had to take SIL here or there so it was a relief to her in a way but I would hear SIL on the phone telling aunt that she could still drive. SILs kids had to step up and make arrangements for their mom.

So your mom is affected by this situation as well as your dad and needs to monitor the Uber driver now from the back seat.

The doctor can support you and that’s helpful if they care what the doctor says but bottom line, your mom needs an evaluation and you need to see what else she is okaying for him to do with ladders and other “honey dos” she thinks he is okay to still do. 79 is not that old and he probably is okay to do many of his routine activities but it’s always good to have an assessment from time to time to try to avoid accidents.

Mom may need a refresher driver course herself if she is deemed okay to drive for now.

Nicole, I think your mother may be focused on what went wrong last time – when he “drove 400 miles away from home”. Yes, monitoring as a passenger could help to avoid getting lost. Talk to mother that ‘next time’ will be different, and it may be a crash with no “guardian angels”.

It’s important that she understands, because if she is still OK to drive, you probably can’t disable the car.