The reason you can't find any rules is simply that they don't generally exist. For example, the only people who have an
automatic bar placed upon their employment in schools are those with convictions for sexual offences against children. In all other cases it's up to the potential employer to decide whether they'll offer the applicant employment. In practice though, no school would employ your friend in any capacity (whether as a teacher, teaching assistant, caretaker, dinner assistant or whatever).
Similarly, there's no automatic bar to your friend obtaining work in a care home but it's extremely unlikely that anyone would offer him such employment.
Even many police forces don't automatically bar people with criminal convictions from employment but, once again, he wouldn't be offered a job. (However, he
is barred from work as a security guard, door supervisor or any other job which requires an SIA licence, since he's currently ineligible to be considered for such a licence).
So, as I've stated, you're mainly looking for rules which don't actually exist. Your friend just needs to be aware that he's extremely unlikely to get employment within (or working alongside) any of the 'caring professions'. Similarly he's unlikely to find employment (at least in the near future) in any job where a CRB check is required.
The latter part of my post here is relevant:
http://www.theanswerbank.co.uk/Law/Criminal/Qu estion755855.html
Your friend should also read up on the provisions of the Rehabilitation of Offenders Act:
http://www.lawontheweb.co.uk/rehabact.htm
Chris