Not fly fishing will leave him mainly to larger lakes, not the remote high country ones, which the larger lakes will generally have people. Make sure when he is buying his fishing license, that he gets the regulations also as there are many, many spots in CO where it is fly fishing catch and release only or other areas that are also non-bait fishing. Also know that rivers can have different regulations depending on the part of the river you are on. CO is very particular about lessening fishing impact in certain areas and it is up to the fisherman to know the regulations and abide by them. "I didn't know" will not help if you are violating the regulations.
Lakes are generally more for bait fishing. If he will be trying to find more remote, high country lakes, those are not really very good for fishing and may not even have fish in them.
Per the area recommendations, around Buena Vista/Salida would be a good area to go to and not too far west.
