A staff Handbook is usually a document that forms part of the contract of employment. Though it may be 2 separate documents they would refer to each other.
A procedures manual explaining or inciting instructions of use, procedure, etc is a guidance manual or similar term. This might be an IT Policy, which would include the business website.
You can make any manual as definitive as you like, use flow charts etc. This supports staff training and provides easy reference so long as it is kept up to date.
Having stuff like this in place shows you are a responsible employer should it ever come to disciplinary or indeed if you are taken to a tribunal for unfair dismissal however, its not the be all and end all.
Id be more inclined to know a) was this part of their original job description (JD) b) if not and was added later, does it fit in with their general terms of the JD? Was it agreed or just put on them? i.e. are they comfortable with this type of work that wasnt in their JD when they started?
If it was an add on task after this person started in the role, did you check and agree it was within their limitations, just as you would have done at the job interview?
If its a genuine case of not getting to grips with a task and the employee is otherwise good at the job which you employed them to do, I would be inclined to ask if a) you are ok to pass it to another member of staff and then b) ask for volunteers.
If it was an add on task, perhaps they think it shouldnt be their job to do it, hence they dont do it correctly because they havent taken ownership.
Of course you can extend and amend a JD, and you should agree these changes, even if mandatory, with the employee. You then write the changes into their new JD which they should sign to agree and accept responsibility.
From then on you would follow a pattern of training and then performance monitoring, before considering disciplinary.
