The simplest solution is to permanently lease the land to her at a rate that recognises the increased value to her apartment from having a parking space as well as an estimate of what the increased levies for her lot would be if they were re-calculated.
The contract would be drawn up in such away as it was permanent and attached to the title of the unit with the amount of the lease rising according to either CPI or the percentage increase in levies for the whole building.
Then reduce the amount payable over the first few months or years to reflect the cost of the work done at the owner's expense.
That way you can set conditions for the use of the parking space that mean any problems that might arise in the future (such as trucks parking there or that sudden appearance of a car port) can be easily dealt with because it is still, essentially, common property. And you don't have to go through the hassle of re-setting the unit entitlements.
I'm sure there are other ways of handling this effectively but, on principle, you shouldn't be giving any common property away just because no one is using it.