That's a commercial site that Bookbinder has linked to, so they'll be after you money.
The relevant pages from the Gov.uk site are here:
https://www.gov.uk/divorce
If your mother will be applying for divorce on the grounds of 'separation', she'll need to wait until she's been apart from her husband for two years (if he's happy to agree to the divorce) or for five years (if he won't consent to divorce).
The fee for applying for a divorce is £550 but there's help available for certain people on benefits or with a low income:
https://www.gov.uk/get-help-with-court-fees