She is entitled to holidays and not to have to work double time beforehand in order to get those holidays. The employer can ask her to take holidays at a different time if the days she has chosen don't suit but cannot just deny her holidays. For example, if the publication comes out at the end of a month, it would probably make sense to take off the last week and first week of a month so she has 3 weeks to do each month's copy.
Are other employees in the same position? I'd be concerned for your friend if she is working significantly slower than other employees - that could end up being a performance issue. But, if it an ordinary employee can't possibly finish 10-12 pages of copy in a month and also have room to take holidays, then it's the employer's job to do something about that. It sounds like the job is generally too demanding (if, as you say, she can only barely get the copy done, let alone the other duties) and, if so, she should raise that as an issue. Note that, as she is only there 6 months, she is in a precarious position vis a vis dismissal. It may be better for her to wait a year before being too adamant about her concerns (unfair, I know). Are other employees having the same problem? Do they take holidays?
I'd be interested to hear if the employer is just saying "tough luck" or if they are querying why she can't get the work finished in a shorter time (and maybe even offering to help her organise her workload!)
Sprite