Or he could just fix the Truenamer first

Words of Creation actually does allow you to find a creatures true name. Hella expensive though.

Actually, it's worse than that. When you no longer meet a feat's prerequisites, you still have the feat. You just can't use it until you once again meet the prerequisites. Since 3.5 doesn't provide decent rules for retaining, you've wasted a feat.

And Exalted feats are even worse, causing you to lose the benefits permanently if you lose them for committing (or becoming) evil. And that's not even getting into the Vows.

If you mean retraining, there are some in PHB 2.
If you mean retaining... I don't know what you mean.

It's arguable that an evil creature who then became good could end up with both feats, and potentially (depending on how you rule it) use both. However, I would have thought that using the Dark Speech was an evil act and would thus prevent him from using Words of Creation, as he'd end up non-good again as a result. Maybe a neutral character could use both, but I think you kind of have to be properly Good to use an Exalted feat.

You would have to rule in direct opposition to the rules:
As has been mentioned before in this thread, the rules explicitly said that only an evil character can use a vile feat.

Sounds like someone needs to dance the Dark Chaos Shuffle.

??

I've been out of the 3.x loop pretty much since 4E released. What is "the Dark Chaos Shuffle"?

A combination of two 8th level spells from Fiendish Codex I, "Shun the Dark Chaos" and "Embrace the Dark Chaos".
Embrace the DC changes on of the target creature's feat to an abyssal feat (among other things) and Shun the DC changes one of the target creature's abyssal feats into a normal feat (and removes the other things from Embrace the DC).

Rinse, repeat; the XP cost of the spells is negligible by the time they are within casting reach.