Mortgage Interest Tax Relief at Source (TRS) can be claimed for the current year and up to four prior years. So she should make sure to apply straight away, so as she will get her refund for 2005.
Being a first time buyer she would be due relief for seven years, & depending on what way the Finance Bill pans out (as in if she is or will be in negative equity in 2011) she may or may not be covered by the new terms, but the important thing is that she acts swiftly before the end of December of this year.
She can't claim for backdated relief using the online form, but she can use this one:
[broken link removed]
Seeing as it's so late in the year I'd recommend that she send it by registered post, rather than Freepost, just to be double sure.