ATOS medical assessor travel costs refund

I have a cl who is bi-polar, has anxiety attacks, cannot be in crowded places or use public transport and a catalogue of physical issues incleeding needing to be close to a toilet at all times. Her partner has a life-limiting condition and can offer little support at the moment. Cl rarely ventures out. She has been migrated onto ESA and ESA50 reflects the fact that she ought to be in the Support Group. She was contacted today by ATOS to attend a medical, so we advised her to request that the medical be held in her home, as she is entitled to do. She did so - and was told that she would have to provide a GP’s letter (not a problem - GP very supportive) and will also have to refund the travel costs of whoever goes out to perform the medical. Now if this was someone “trying it on” I could understand that ATOS would want to be reimbursed if a cl could genuinely get to the medical but it is clear from ESA50 that this cl cannot get there. Since she cannot afford to repay anything (and ATOS was unable to give an indication of just how much these costs might be) cl now feels railroaded into attending the medical. This will cause her huge distress physically (she was discharged from hospital yesteray) and mentally. She may not even make it there.
Does anyone know where I can find any regs which state that someone too sick to attend a medical has to pay for the assessment to be held at home?

No-Ive sat in on a home WCA with a vulnerable client for whom we obtained a GP letter and there was no charge. MP’s letter?

Yes, MP’s letter being done as we speak, also complaints letter. didn’t think there were but just in case ..................... Unfortunately cl not able to come to Bureau and ATOS won’t speak without cl, so can’t intervene.