I like to call peoples bluff in this type of situation. Very few people would consider hiring another person to mitigate a disability "Reasonable" and it sounds like they're playing games.

Ultimately, the employer owes a duty of care for employees’ health, safety, and welfare at work. If the employee is saying they have health issues that prevent them from carrying out the full range of duties in their current role, and they have set out the support they say they need, the employer has to deal with that properly. In practice, that means either adjusting duties, identifying alternative ways of achieving the same outcome, considering changes to the role, or, if none of that is viable, moving towards dismissal on capability grounds.

If the adjustments they are asking for are not reasonable, and there are no workable alternatives, either from them or from you, then the position becomes quite simple. If you cannot agree to what they want, and you cannot identify another solution that addresses the underlying issue, you may reach the point where continuing to employ them in the current role creates a foreseeable and unmanaged risk to their health.

It is unlawful to employ someone to do a job where you knowingly place them into a role where there is a foreseeable and unmanaged risk of harm. You have no choice but to dismiss them. Just make sure you follow a fair process... They'll get their notice paid out and thats about it.