Lots of people think they have curvature of the spine but are in fact completely normal. That said, we all have twists, turns and kinks, very often this has been explained to the person at some time and they've worn this explanation like a set of clothes, affecting their self-image, confidence, pain perception, everything, and usually unnecessarily.
There are also a lot of mild scolioses out there, that cause very little trouble, if massage is harmful in these cases it's probably not because of the curvature.
There is the occasional severe scoliosis, but you would recognise it, and it's not necessarily harmful to massage, but go easy, forcefully undoing every tight-bit can be couterproductive, as these are often areas of support or compensation.
So caution is required if it is severe or they look unduly crooked, sudden angle changes or areas of severe pain or inflammation, if they are unwell in other ways, or if they report worsening.