A player recently approached me and asked if he could create a character with both the fire and cold subtypes. I thought I remembered something that said you couldn't but I cannot find it. if you could please help, and provide a page number I would appreciate it.
Yes, a single creature can have both the Fire and Cold subtypes.
It's actually pretty trivial at higher levels. There's a pair of spells from Frostburn and Sandstorm which permanently add the Fire and Cold subtypes to a creature. Each gives vulnerability to the other type, but immunity plus vulnerabilitiy still equals immunity. They are also both Instantaneous duration, so they can't be dispelled, and the effects work just fine in an antimagic field. The two spells are Mantle of the Icy Soul and Mantle of the Fiery Spirit. A scroll of Icy Soul will cost 11125 gp, and a scroll of Fiery Spirit will cost 18000 gp.
You may do so. There’s nothing to cite on the matter: there simply doesn’t exist a rule anywhere saying that you can’t. In fact, there’s a certain Barbarian “style” (can’t really call it a build, it’s just a pair of feats) that revolves around taking both Blazing Berserker from Sandstorm and Frozen Berserker from Frostburn to get both subtypes while in a Rage.
Ultimately, it’s pretty nice to have immunity to both Cold and Fire damage, but neither immunity is particularly hard to come by in the first place (particularly fire). The Blazing and Frozen Berserker combo is the easiest way to do it, and at two feats it’s already pretty expensive.