Seriously?
Sorry, but sell the car and buy a beater. If their financial position is that poor then they really should be walking or riding the bus.
Im sorry, but it sounds like vanity caught up with stupid choices, and they should move down in car.
I know that's not the response you want, but youre not doing her any favors continuing unsustainable lifestyle by giving her crutches.
Personally, I dont know that an engine failure on this engine is any more expensive/tougher than any other, but I think it is false economy to stray from the recommended oil choice... And time and effort will expand at cost versus doing it right to begin with.
Teach her where to get the parts for good prices and how to DIY the change and cut costs that way. That's just smart economics. Doubt the ML even needs to go up on ramps...