It never ceases to amaze me
(a) how many questions are asked about how to write a basic letter, and
(b) how often answers are supplied pointing to templates to help
There may be some templates available and someone may suggest one but,
personally, I think that she should just write a letter in her own words
stating why she wants the job and what she could offer to the job. A good
individual letter will show her in a far better light than any standard one.
As it is an internal vacancy she may be able to get some inside information
by asking around her colleagues.
As an aside, I once interviewed a chap for a job, more or less as a
formality - I had already decided to offer him a job based on his superb
application letter - and I was not disappointed. I just wish I could
remember what he'd written
