Thanks for asking your question on JustAnswer. My name is XXXXX XXXXX X'XX do my best to answer your question.

First of all you need a new mechanic. This is ridiculous. This mechanic should have AT LEAST taken it for a test drive. You can't find out if a car is going to overheat or not with it just sitting in the shop. A car can run for days and not overheat if there is no load on the engine. OK, I'll stop, this guy doesn't know what he's doing though.

Now to your question. The first thing that I notice after reading your post is that the air coming out of the heater was cold. This tells me that the coolant level was low. After doing this type of work on a cooling system it is actually fairly difficult to get all the air out of the system and get it full of coolant. The air coming out of the heater would have been super hot if the coolant was full and circulating through the heater core.

Just from your initial post there is no way to tell if there is any other problems with the cooling system since it wasn't properly fixed in the first place.

Does this answer your question?