After the 26 week Statutory Maternity Pay, you can take another 26 weeks off as Additional Maternity Pay but only the first 13 weeks are with pay. There are “qualifying conditions” but I don’t know what those are, might be worth checking out the direct.gov website for that.
It would be wise to check out her company’s rules/policies/procedures as there should be a condition as to how much notice they require if you were to change the date of returning back to work – if they have stipulated a time limit which is longer than the notice you’ve given, then it may be very difficult to do anything about it.
In terms of taking holiday at the end of maternity, I think there is nothing to stop this aslong as adequate notice is given. Infact, the company I work for insist that all holiday is taken at the end of maternity leave!
If nothing else, she would be entitled to a copy of those documents for you to then use in a case against them if it did go to tribunal.