I think with undead it was established that the Light will come to those who call upon it. Someone has to have a strong conviction or will in order to make that call. It’s less that the Broken can’t because of the red pox but more that they have mentally been so broken that they believed that they /couldn’t/. It’s more of psychology based type thing.
A reformed man’ari could use the Light because they have a strong conviction or will. And therefore can call upon the Light. It may or may not hurt them. Depending on the Man’ari. Unlike with the undead, that is going to burn. And they may or may not be on fire.