There's a lot of reasons it's heating up. I assume he has the AC blasting and no oil changes ever . He should run the heat with Windows wide open and take some off and check the thermostat those models always lose thermostats .
I’m not much of a mechanic, but the thing he just did with the coolant doesn’t seem like a good idea.
Just saw this thread. I didn't read all 10 pages yet but I will. This is a pretty common problem (I'm not saying it's his only problem, but it may be one of his problems) so I kinda want to make sure other car owners here, who may not know it, can see this first.
What Chris describes sounds like air in the cooling system. When you leak, air can go into the same leak where coolant comes out. It'll obviously float to the top, where it will stay. The cooling system being partially not coolant will lead to uneven cooling, and the temperature sensor's reading may not be applicable to the whole system simply due to the heating being uneven, so the cooling system may kick in late for some parts, which eventually may overheat. Also the temperature sensor's reading may spike periodically, again due to the uneven cooling.
Edit: Chris has mentioned both symptoms above in the video. He said the mechanics couldn't fix all of the leaks previously, then he filled the coolant reservoir "yesterday," meaning the car would have had time to leak some more and develop more air bubbles. He also said the temperature gauge shot up "to the H immediately," so it spiked. If he is to be believed, this is the most likely problem with the car. All the other stuff other people are saying about head gaskets and thermostats breaking, I also agree, because it's only a matter of time even if those haven't happened yet.
What Chris did with filling the reservoir isn't a particularly good idea, but not bad either. It's normal to top off the reservoir, to the marked "full" line, only while the engine is cold. There is nothing wrong if you do that, it just doesn't really help the problem right now. (Chris probably wouldn't have the patience to wait for the engine to cool before filling the reservoir, or he may not know a "full" line exists. It's probably not too bad, coolant will probably just overflow while he is driving one of these days and cause a lot of steam suddenly, scaring him LOL)
To get rid of air pockets within the cooling system, park the car, preferably at an incline, so that its radiator cap is the highest point of the system, uncap the radiator, then just run the engine until it is at middle of the heat range, and slowly watch the air bubbles escape at the cap. Simply top off the radiator from the cap as more air escapes.
There are probably countless videos on YouTube that will go into further specifics if anyone needs more details for their own cars. Some cars might even burp themselves from just driving them. "Burping the radiator" would be a search term you can use.
No, don't bother buying his shit specifically for this. It costs $0 to fix unless another problem comes up, and even then, a new radiator is like $50 on ebay, depending on the car, plenty cheap enough for him to afford.
Edit: LOL he dropped it off at a mechanic? Yep he's fucked. They are going to recommend a whole new radiator most likely, because Chris said they already tried and couldn't fix all the leaks. It is absolutely a labor intensive thing because of so many hoses and wires to unplug and coolant is going to get everywhere if a hose flings around and they have to wait for the car to cool down before working on it. The mechanic, even if he was honest, will still be within his right to charge a bunch of labor. It's not particularly difficult, but it can get time consuming. Hope that shop accepts his tard card.