It actually needs to be a bit less than 4000 characters- 4000 characters properly spaced will hit the line limit first! I don't know if that helps at all!
I would get her to write something as soon as possible- it doesn't have to be especially flowery, or even that good (as long as the spelling and grammar is okay). Presumably her school/college has an internal UCAS deadline relatively soon, as they need time to check the application and write the reference?
Often, the PS is not the deciding factor as to whether an applicant gets an offer BUT if it doesn't get written at all, her application can't go off- and then she won't get equal consideration with other applications.
As a very, very basic structure, I'd suggest something like:
Paragraph 1: Why does she have a general interest in the course/area she wants to study?
Paragraph 2: What specific things is she looking forward to in terms of studying the course at uni level? What about her current study of the subject has she enjoyed?
Paragraph 3: What skills/experiences does she have that make her suited to the course e.g. books she has read, exchange trips etc.
Paragraph 4: A little bit of general extra-curricular stuff e.g. hobbies/clubs.
Paragraph 5: A conclusion, summarising her interest in the course. It could also mention her plans for after uni.
Some of these sections could be longer (not the general extra curricular stuff, though) and some could be a bit shorter- they don't need to be a long paragraph each. These are just general prompts that might help her come up with ideas.
As long as she uses formal language, it doesn't have to be flowery- she can talk about herself in plain English!