I don't think any doctor can guarantee a "cure" for metatstatic breast cancer. To date, there is no known "cure." Most of us, even those of us who have gone a long time without evidence of disease, know that the proverbial "other shoe" could drop at any time, as it has with your mother.
I haven't heard of photodynamic therapy as a breast cancer met treatment, but I am by no means an expert on the subject. I would think that most doctors would talk about treatment in terms of expected months or years that could be gained by a treatment.
We have many women who come here who have survived with mets for many years, and some doctors prefer to talk about metatstatic disease in terms of a "chronic illness," rather than a life-ending illness; and while I do say that your mom could have many good years left, I think you should be aware that there IS no cure.
My hope for my friends with mets is that current treatments give them the time it takes to FIND that cure.
Perhaps someone with more knowledge will be able to answer your question about the success with that particular treatment. But beware of anyone who promises a cure.
BEV