On the one hand, there is certainly some can-of-worms potential, as pointed out by UmbraLux.
On the other hand, for what you're doing, that probably doesn't matter. See, the only time you'll run into problems is if someone starts trying to hunt high numbers to the exclusion of all else - and that's really a meta-game problem. I'd suggest allowing such a thing, with the caveat that you may want to change it later, after seeing how it works in play. (And, given the high power of spellcasters in general, you may very well want to change it later, even if it's "balanced" against someone with evocation.)
That said, there are a number of ways to boost lightning magic without actually buying up specializations.
I'd start by looking at Kemmlerian Necromancy; a Lightning Mastery "sponsored magic" based on that would give her the go-ahead to do all sorts of fancy things with lightning evocations (turning on lights, ball lightning, etc), above and beyond the normal evocation attack / block / maneuver options.
I'd also look into Breath Weapon (though with the note that it's probably overpriced - at a minimum, I'd switch it to running off of discipline or conviction rather than weapons), to give the character some ability to throw lightning around without mental stress.
And, third, I'd look at Evocation again. Ok, your powers are all lightning / electric based. That could give you some access to Air, Earth, and Spirit - lightning itself, plus some ability to manipulate weather / wind, plus magnetic effects (telekinesis vs. metals), plus some mirages & illusions (deflecting light by heating the air, creating light), etc. And if you run into a situation where your particular focus limits you more than evocation would? Well, that's a perfect time to suggest a self-compel to the GM...