Hi there;
A reputable resource for finding DBT therapists in your area is the
PsychologyToday.com "Find a Therapist" page. You can start by filtering by location, then click "Filters", scroll to "types of therapy", and select DBT.
It's important that people don't feel pushed into therapy by someone else, that they feel trust and connection with the therapist, and that they're going for themselves, not due to ultimatums, guilt, or what other people want. (Not saying any of that is or isn't going on here -- just speaking generally)
If your exGF can be the one selecting a DBT therapist and making (and keeping) appointments, and if she feels like the T understands her and is on her side, chances are better that there can be some benefit.
Therapy can take a long time when it's about really fundamental stuff (FOO issues, things from childhood, personality organization). For example, for me, it's been "quick and easy" to process finding our housemate dead in our home... it's been
much more difficult and long-lasting to work with internal feelings of shame coming from my childhood.
I would be patient. This will take a while. It's great that she's interested in DBT; I would not expect a "results in just six weeks" kind of outcome.
Does that start to answer your question?