There’s not enough known about this car to make much of a guess as to what’s going on but just offhand it does not sound like it’s going to be trustworthy at all.
Some sealers may help a little in stopping various low pressure leaks from coolant passages, etc but when it comes to a combustion chamber breach it’s a lost cause in my opinion and there is certainly no way I want to be on the road with an engine stuffed full of Goop 'Em.
You state the radiator is brown and frothy. This may or may not be a sign of a bad head gasket. They replaced the radiator before but that doesn’t mean they flushed the cooling system clean.
The first step is determining if a head gasket is actually bad. Cooling system pressure test, hydrocarbon test, compression test, etc are a few of the methods used to determine this. The odds of a cracked block are very very slim. That is a diagnosis that is tossed around quite a bit and is seldom ever the case.