I'd say holy and shadow share more synergy than disc and shadow right now since holy and shadow tend to get more value out of crit, spellpower and spirit. Disc builds seem to benefit more from haste, int and spellpower (or atleast I have).
I'm not sure why you say Disc has slower cast times. Penance will have its first bolt hit almost immediately (travel time) when you hit the button and you have talents for 5% haste right off the bat with an additional 25% haste grated through borrowed time for one spell (or two if you use penance first) every time you use your shield.
I won't deny that an equally geared holy priest will have the edge on an equally geared disc priest in terms of spell power. I will point out that the disc priest will have a larger mana pool and thus benefit more from rapture and replinishment than a holy priest will benefit from serendipity and replinishment while having the same I5SR/O5SR regen.
Anyways, to the OP, tell your girl to pick which spec she finds more fun.
http://www.wowhead.com/?talent=bVcbuhxtMxsVfRt0xxZh is the build I run but with 3 into spirit tap instead of inspiration.