The beginner area is where you will start and depending on how fast you pick things up you may even advance to another slope. I don't know, I have never really seen too many heavy people on the slopes. If they pop right up skiing will only be about 3 times harder. But seriously, like any other athletic activity, I recommend staying within you physical ability. They may need to take more breaks.
Skiing puts a little more stress on the lower body, especially the knees.
Finally, skiing on my own skis is a lot different from skiing on rental skis. A ski shop can give you some ideas on waxing yourself, plus they can do it for you. The boots are not going to break in as much as your feet are going to become conditioned to the boots. Make sure that you are only wearing one pair of ski socks, don't have a bunch of other layers packed into the boot, remove any wrinkles in the sock before buckling the boots. Wear the boots around the house, but be careful on your mothers fine floor covering.