Taking fuses out to fix the heater? I think that your sister's boyfriend knows even less about cars than I do ;-)
You'll gather that I don't claim to be an expert but I've had Fiestas with heating problems, so I'll have a go at answering your question:
The first thing to check with a faulty heater is the coolant levels. (Insufficient coolant in the radiator system will mean that the heat exchanger won't be able to work efficiently). Then it might be worth trying to flush the coolant system through (to try to shift any blockage in the system). Otherwise, a new heat exchanger is probably needed.
You've not said what happens when you try to start the car. If absolutely nothing happens, there's no power getting to the starter motor, so you need to check that the connections to it. (Also, double-check the fuse).
If you just hear a click from the starter motor, it's probably jammed. Whacking it with a hammer sometimes will sometimes free it but this should be regarded as a short term fix and the motor should be replaced as soon as possible.
If the starter motor turns over very slowly, it's getting insufficient power from the battery. (If so, it's probably a fault with the battery but the alternator can't be ruled out).
If the starter motor turns over normally, then there must be a fault with the ignition system. This could be a problem with the points (unless, of course, the car has electronic ignition), moisture ingress (e.g. damp under the distributor cap) or (among several other possibilities) faulty HT leads.
As I've indicated, I don't claim to possess any great knowledge or expertise but I hope that something here will point you in the right direction.
Chris