If you spend more than $2k on repairs to that car, that is money that is like lighting it on fire. I certainly understand spending money on routine maintenance to keep a vehicle on the road, but at 13 years it owes you nothing, and to spend more on repairs than what the vehicle is worth is insane.
You would be better off taking the $9700 in quoted repairs and putting that towards a newer/better vehicle, and see what you can get for a trade in/sale of the Murano. Even with the insane prices in used/new vehicles now you would be better off than sinking that much into a 13 year old vehicle with 100k miles.
This is somewhat debatable. Without closely scrutinizing his repair quote, I'd conjecture an independent shop could do all that for $3500. Assuming it all needs to even be done. If his drivetrain is sound, 100k miles on the odo isn't even high mileage by today's standards.
Meanwhile used car prices are up a good 37% year over year. If he wants another crossover, he can forget about finding anything for $10k, and his replacement cost could be $20k. And he'd be buying in at an inflated market peak, so his depreciation curve is gonna suck once the supply chain returns to sanity. Right now is a terrible time to buy a car.
The solution is obvious. Unless he lives in a small town, find a good mechanic. Although traditionally that wasn't that easy, Yelp reviews help a lot. If he lives in any metro area at all, good indy mechanics exist. Fix the things that need fixing; otherwise just do routine maintenance until repair costs become excessive.
Personally I only do the simplest maintenance tasks like replacing filters and bulbs, so advocating for DIY auto repair isn't for everyone.