If it's running off of batteries (more digital thermostats run off of batteries it seems, even my analog one does nowadays, I think the batteries run the temperature reader, which is very, very sensitive and fast to react) then it's not a loss, it's just a wiring issue.
If he's got 12 volts running into the thing and it's functioning, he can get the fan to turn on. The big question is: is the thermostat a cooling/heating or just heating item?
If it's just heating, you'd have to build a relay circuit to basically reverse it's function. It might be easier to buy a heating/cooling one than try to wire up a relay.
I think this is the most important thing that some people miss. MAKE SURE YOUR THERMOSTAT IS RATED TO COOL (ie it can turn on the AC). It's not nearly as useful if it can only heat.