Thanks for the tips. I'm going with Holy Priest. I found a decent website to help guide me in addition to the one posted earlier:
http://www.HealingWoW.com/
Just a heads up holy priests are currently the worst of all the healing classes atm and it will most likely stay that way for the remainder of this xpac (so at least until the end of this year at the earliest). Priest is a good option overall though. Disc is the top healing spec atm with holy pallys next and druids/monks after that. If her group is a 10man I'd suggest a holy pally as they are probably the easiest to play. A paladin would also give you the option to try both tanking and dps if you had the itch (so would monk). A bonus for monks would be the bonus to exp gain while leveling so that would go quicker. Then again all of this is pretty moot if they are a really casual guild with no real aspirations to clear normal SoO. In the end though it is all about having fun.
Some follow up stuff:
Do you know what type of healers you will be healing with and if you are doing 10 or 25?
If you are looking to be the most powerful healer that you can possibly be in 10 or 25 man raids I would completely suggest a disc priest as they are far and away the most dominant.
If you are doing 10s and the other healer is a disc priest I would suggest either a resto druid or a holy pally.
If you are doing 25s or 10hm a resto shaman is a great choice.
Whatever you do I implore you to not go holy priest as it will only end in frustration as you watch everyone die because the class cannot efficiently group heal or single target heal. You'll burn through your mana and still have dead people.
Next part (yes it is raidbots, flame if you will, only using it for its generality of data)
normal mode data is quite similar
10hm healer data -
http://www.raidbots.com/dpsbot/Spec_Score/10H/100/14/60/default/#0000000000000000000000000000111111
25hm healer data -
http://www.raidbots.com/dpsbot/Spec_Score/25H/100/14/60/default/#0000000000000000000000000000111111
Short version of the data is that holy priests are doing half the healing of the other classes/spec regardless of boss, raid or difficulty.