We thought gently explaining that of course she wants her daughter to succeed. and that her daughter IS brilliant creative talented-- ( we believe this) .....think what her daughter's life would be like if she only could agree to therapy.......
I thought so too. My BPD mother was brilliant, charming, beautiful. If only...
However, she had an enabler- my father- who fortunately had the financial means to provide for her and yet, even so, there were financial issues from this situation. My father was a part of the dynamics in the relationship. Your son is too. Yet, we don't have control over another person.
Because of the denial and projection, and avoidance of shame, my BPD mother could not consider that she may have had issues to work on. She did have therapy but was resistant to it.
I don't know if you have considered 12 step groups and/or counseling. 12 step groups like CODA help with these dynamics, even if there isn't substance/alcohol abuse because they help families manage enabling behaviors.
Of the three enabler situations, there's only one you have control over and that is your financial support. As hard as it is to not help your son financially, and in other ways- enabling in any way can prevent the person from experiencing the consequences of their situation- the consequences that could help motivate them to change.
If you feel it's important to you to try to speak to the mother- then do so, but also know that the reaction may or may not be what you'd hope for. Even if the mother did encourage therapy, the daughter may or may not agree to that or be willing to work with the therapist.
From my own experience, therapy and CODA/ACA groups for me was most helpful. I would recommend it for anyone dealing with these kinds of dynamics.