Your father has reason to be concerned. Connecting a battery charger to wire is known as "shorting the circuit", which is almost always bad.
It could work. If the equivalent internal resistance of the battery charger was high enough to keep the current flowing through the wires low enough to keep the wire and/or charger from burning up.
The worst case scenario is that you burn up the battery charger. The acceptable scenario is that you destroy the wire on your frame or over heat your foundation. The best case scenario is that the wire heats up just the right amount to melt the wax foundation in a reasonable time frame.
What you really need in a situation like this is a way to control the amount of current/voltage applied to the wire. Or even better, a temperature controller that allows you to set the temperature created by electrifying the wire.
Using a car battery is nice because the car battery is not likely to blow up before your wax is melted (which probably won't take more than a couple seconds or less). This is likely not true for the charger which usually will contain electrical components which don't handle being shorted very gracefully.
My university education is in electrical engineering. If I were in your shoes and the charger was just sitting around wasting space, I'd give it a try and hope for the best. If you ruin the charger, then, try something more intelligent the next time.
Whatever you do, video tape it and post it on the forums!