Most anti-epileptic drugs are "enzyme inducers". What we mean by this is that they tend to up-regulate the liver's 'detoxification enzymes'* (specifically, elements of the cytochrome P450 family). The liver thus metabolises these more quickly.
Now, what happens if you give this patient a different drug? Well, if it is not usually detoxified by the enzymes that have now been up-regulated, nothing much changes.
However, if it happens to depend on the same set of enzymes that the anti-epileptics do, things get interesting. Since the enzymes have been switched to a 'more powerful' state, they will metabolise the new drug more quickly, and so its levels in the body will be lower than they otherwise would have been.
To compensate for this, the doses of certain medications have to be increased in patients on treatment for epilepsy.
* Actually, 'detoxification' isn't that accurate, since some of the transformations that a molecule undergo in the liver can even render the substance MORE toxic than before. 'Metabolism' is probably preferred.