Your friend is going to spend more cash anyway when his PSU breaks and destroys his video card on the way down, possibly the motherboard too/instead.
Potentially a whole lot more.
Sure, it would be nice if you could undervolt the whole system by 50% and gain FPS in the process, but if the highest ranked people on TH are suggesting something it is probably because they know what they are talking about.
If he wants a bargain basement priced PSU he can get an Antec EarthWatts 380w for $42 which is still light years ahead of the current Apevia one.
I would much rather see them with an XFX 450w for $47ish, though. That may not handle a 6990, but it would handle the current specs easily and anything the friend decides fits in his ultra restrictive budget later.
If he won't cave, ask him if he would buy a Ferrari except to save a few dollars off the sticker price he would ask the dealership to put a Toyota Echo engine in it instead.