There are a number of issues here.
1) confirm that is a strata title with your mother
2) a handrail can be installed at the OC expense, however it needs to go to a general meeting for approval as it is an addition to the common property.
3) A precedence has been set by allowing additional paths to be constructed for some owners with OC funds
4) the proposal for the handrail could include general safety and slip concerns not only for your mother but for any visitors.
5) I understand that the Government do provide some assistance for elderly folk. A Occupational Therapist could visit on a recommendation from your GP.
6) If there is no bylaw in the scheme, and your mother has to buy the handrail your mother may have to provide one ,accepting any future maintenance for the handrail.
Good luck!