Minor settlement cracks are common and are usually treated by the application of a flexible sealant prior to carrying out normal regular painting (as part of the normal preparation for painting).
It follows that any internal painting of the unit is the lot owners responsibility and this should be taken care of by the lot owner with their standard maintenance of their lot.
However – take care not to always label the cracks as cosmetic as if they are constant and continuing to get works (whether in one unit or more) they could be an indicator of other significant issues. In this circumstance, ignoring the request of the other owner could lead to liability issues further down the track.