As 26/27th December are in the 'high' season, I wouldn't expect many sales to take place. It's possible, though unlikely.
Have your friend check out
www.netflights.com, it is a consolidator here and she can get a good idea of flight costs. I'd also look at expedia.co.uk, as they have their own 'deals' and display essentially published and consolidator fares. For example, plugging in 27th Dec-12 Jan came up with £448 as a 'special fare' (read consolidator) on UA. A published fare of 520 is also available. On netflights, they had a fare of £380 on AA, but it requires transferring from JFK to LGA (which by time you add the cab fare and hassle of transferring makes it worth the extra, IMHO).
I checked out
www.thomascook.co.uk thinking they may show some charter options, but they popped up £380 on NW via MSP. She'd fly from Gatwick (and gasp, shock horror fly on NW), but that's still £70 to spend on champers for New Year!
I'd say book now and be happy in the knowledge she'll have a white New Year.
Edit to add: Looking at the flight timings with NW, they do involve a SIX hour layover going to Calgary and a SEVEN hour one on the way back!
[This message has been edited by LHR Tim (edited 09-19-2002).]