No cheating on stats going on at all, the issue is more like that having optimized character, he also optimizes the encounters and often use enemies of higher level than us (a CR 26 dragon at level 17 happened among other things, but on average the CR of a single monster in an encounter with a bunch of them at higher level is at least +3/+4 the party level), most of them have 2 o 3 strong saves that even with a DC around spell level+30/32 (dragon aura (cold) for a +4, greater/cold focus, the armor from the website that gives +2, +10 int, academy sorcerer from Dragonlance for another +4, etc) they'd save arond half of the time, and in my book having a 30/40% chance of failing with a spell, is a trap
In my albeit limited experience with DMs I found that most of the time that's the way they deal with optimized parties, so I tend to avoid going that way just to be safe.
On the other hand, huge heaps of damage, and crowd control that has no saves or really high saves (like ones depending from damage/power attack/str, for example Boomerang Daze) still work quite well