Actually i would recomeend any raid take one dis with them, does not matter 10 or 25. Shields have saved so many lives...
And yes - atonement still lives and shines on a couple of encounters like alysrazor/ryolit.
Having a disc priest in any raid is extremely potent, because it's a niche healer. This is even more true in 10-man since the portion of the raid that can be shielded is much larger. Also, in 10-man having an additional raid cooldown (PW:B) is very useful.
For HM's in current content, having a Priest who will only play one of the 2 healing specs will gimp you on some fights. Holy is better than Disc on some fights, and vice versa. If it's a case of needing a healer to pewpew on 2 healer fights, then the rDruid could just as easily go Boomkin/feral spec. But if it's more a case of "I don't wanna lose my shadow spec as I need it for my dailies" or w/e then it's time to choose dailies or progress (or respec alot).
My opinion is that any priest healer wanting to do HM content really has to be able to swap between holy & disc, and if I were raid leader not doing so would not be an option. Granted it's of course a call for you and your guild, but if you want to raid seriously then the respecing is part and parcel of progression raiding. I am kinda interested as to why her losing her shadow spec doesn't seem to be an option? .
Assuming she can play both specs equally I'd say if I have to choose 1, I'd pick Disc as it offers more utility than Holy.
Our priest does have the ability and experience of both healing specs.
I think, judging from your replies, that I'm going to see whether there's anyway she can roll disc/holy (she is already an officer and the GB is more than happy to pay respecs). But I do see here that disc, for the HMs, seems to overtake holy by miles, and if it really does have to be one spec or the other, then disc it will be.
You need to roll whatever spec is best for the fight, especially in a 10m. There is no one-size-fits all healing spec for a Priest. Blizzard has said quite often that Priests are the premier healing class and both specs should be equally considered. To say that Disc (or Holy) is better than the alternative all the time is simply incorrect. Barriers and shields are nice on some fights. Raw healing output is necessary on other fights. It depends entirely on the encounter.
The only time I would consider locking yourself into a single spec is if you are in a 25m, and have elected to take the output loss in order to provide a raid benefit (mainly pain sup or GS).
Disc scales better and brings more to the table. Also with the current spirit cloth arrangement Disc will benefit a lot better from the heroic gear, because most of a lot of Holy's BiS 378 gear is exclusively from Valor points, and the 391s suck itemization wise.
This is from the perspective of a Holy Priest, who raids alongside a Disc Priest in 10 mans. Dat barrier mang, so gewd.
Yeah we carry some of our healers thru the dailies as well, apart from being quicker in a party it's more fun, so it's win win . And finding a guild dps is less hassle than constantly having to fanny about respecing etc.