Am not sure from your post whether the owner has lodged a fully scoped proposal to install the chairlift. Until they do, you don’t have to do anything.
Any such scope would include detailed specifications regarding the type of chairlift, how that fits with the type of stairs you have, all health and safety considerations etc. This would include matters such as fire and emergency issues – if there is only one staircase this would be a key factor and it is possible that would prevent the installation anyway. Council approval may be required.
The owner would also have to accept responsibility for the installation, i.e. the cost and ongoing maintenance, and removal. In those circumstances the OC would be reasonable to ask for some form of financial compensation.
Essentially the OC would be granting an owner a form of licence to use the common property for their benefit, which on any analysis would disadvantage other owners.
Once the Committee is provided with the proposal it is then up to them to consider it, and either recommend it or not recommend it to a general meeting. Personally, I think it is a big ask and if that proposal were put to me I would say no. It is a very difficult situation when someone loses their mobility and can no longer live in their home, but it is not uncommon. I live at the top of four flights of stairs and recently broke my leg very badly – I have been able to leave my apartment three times in 7 weeks. I have always recognised that if my mobility were affected permanently then I would have to move.
If the OC refused consent for the installation the owner could take action on the basis that the refusal was unreasonable, but they would have to prove that in the circumstances it is unreasonable.
There has been a case on this issue, where the owner did succeed but each case is different, and that was a very different type of building, and the lift was a permanent installation that did not impede the stairs etc. (sorry can’t find the case)