Permitted Work when on ESA and PIP

Hello, I am helping an abused pensioner who is currently going through an acrimonious divorce settlement. The court case re financial settlement is in a few weeks time.
Can you tell me what are the rules for permitted work please?.
The ex is in ESA support group, and received PIP at highest rate for both care and activity. Both awarded due to brain tumour around 2017, treated with radiotherapy in 2017.
Tumour now seems stable and the ex appears much better health wise, though likely not well enough to go back to self employed design work. But ex now easily manages to run Airbnb from home, and works most days in the allotment.
Originally claim for ESA and PIP was possibly granted under exceptional circumstances rules. Claimant currently runs Airbnb (on hold because of covid of course) and also receives income from a student flat.
Is this allowed under the Permitted work rules please?
I am not sure what the rules are, I understand you could be allowed to work up to 16 hours a week, and earn up to £140 a week.
Can you clarify if the permitted work is likely to be granted to someone who argues ‘being employed would cause significant harm, and they need to carry out the Airbnb and allotment to keep sane? Under what circumstances is work permitted?
Can you clarify if the permitted work rules only allowed for someone who intends to return to work, and to ‘test the water’ beforehand ?
I’d really appreciate anyone’s help, it is quite urgent please.
Sincere thanks

Thank you for the link and your reply.