I have two contrasting feelings about buying cheap .... at the bottom end of the market there is some terrible rubbish such as all the full suspension bikes - chronically heavy and really poor materials, whereas you could buy something basic that probably would still not be made with quality materials that might get you on a bike and then thinking about what you actually want. It was a cheap horrible BSO that got me on a bike ... and had me wanting that second purchase. And one friend did try very very gently (too gentle in retrospect) to buy a different bike.
I don't have the problem with buying a second bike later ... how many on here only have the one bike!!! But I would at least try quite hard to direct him away from the worst of the purchases (so anything full suspension/disk brakes etc) he could make and to something that has a bit of potential to improve. Get him to pick up a really heavy bike and a light one, and explain that the metal that is used for bits of componentry such as the noodle and other bits on the brakes can be made of very soft metal - steer clear of those.