My cousin has a 2005 BMW 745LI and he says that it’s burning oil and the brake sensor always turns on, the gas gauge won’t work and some times the car stalls. Should he keep the far or get a new car?
If he likes the car it will probably be cheaper to fix all those issues then to buy a new 7-series BMW.
Sounds like he’s rationalizing why a new car makes sense–in which case it won’t really matter because his mind is made up! 
It's up to him. Any dealer is going to take a look at it and deduct their cost of fixing those things that need to be fixed from whatever price he my really be offering.
Note: When you are buying a car and have a trade-in the dealer loves to mess with your head. If he things a high trade-in value will motivate you, he may add (or not deduct) an extra $1,000 dollars to the price of the new car just to give it back to you as trade-in.
Your cousin will have to ask himself/herself if the new car is worth what he is going to end up paying for it. I suggest he compare the total cost, including down payment and all monthly payments and not the amount that shows on the sales documents.