A person may leave their house "to take exercise outside . . . in a public outdoor place, with one other person who is not a member of their household, their linked household or their linked childcare household".
[Regulation 6(2)(c)(iii)]:
https://www.legislation.gov.uk/uksi/2020/1200/pdfs/uksi_20201200_en.pdf
So your 'more able' resident CAN join their friend for exercise.
Further, paragraph 3 of Regulation 6 provides that a carer (for someone who needs full-time care) is not included when counting the size of such a group so, for example, you could accompany one of your 'less able' residents on a walk with one of their friends (if those residents need you with them when out for a walk), with the three of you still remaining within the rules.