I used to write wills for a living. My advice is to forget about using any forms; you don't need them. You EITHER need a plain piece of paper OR a solicitor!
Get hold of a copy of this book. (It's in many public libraries):
http://books.which.co.uk/products-page/acq3/wills-and-probate/
Read all of the relevant sections, then re-read them.
If you're confident that you can help the lady to draft her will, do so and then get her to sign it in the presence of two witnesses (who aren't beneficiaries of the will).
If you're less confident, still help her to prepare a draft but then get a solicitor to 'tidy it up'.
Chris