Black Spots on Crown of Thorns Leaves?

I gave a crown of thorns cutting to my mother a few years ago, she lives in Florida. This last year black dots have started appearing on the underside of its leaves, as shown in image attached. What is causing it and how would she get rid of the problem? I don't want it to die out on her. My suggestion was to take it outside and let it enjoy the sunshine, which has been a challenge for Florida this year.