Air Conditioning - Need general advice I guess

Alright, way out of my realm here but my folks just called and said their a/c has been out for 2 days. Said the repair guy came out today and said their 4 ton trane compressor 'blew out'. Said the only thing it needed was a new compressor. It apparently turns on but the freon has escaped from the side. He said the compressor needed to be replaced and is looking for parts.
Sorry that is all the info I have and I will call them tomorrow and see if they can get the numbers off the unit but wanted to get a head start to make sure they weren't getting shafted.
They just bought this house but there is no warranty or anything, chalk it up to the crap happens rule and this is an older house.
My question right now is: He mentioned an 'aftermarket' compressor. Do they count on the aftermarket compressor? Is it worth it to go with a replacement trane compressor? Is it time to replace the whole outside unit?
I know this isn't much to work with but just wanted to get a head start.
My guess would be to replace the whole unit with a newer unit but I don't know what his quote for a compressor is.

It's always good practice to stay cool by a fan while you get 3 quotes on the job. Let competition expose any "shafting" attempts.