Hi Joanie
BPD in young people is very difficult to diagnose, and often misdiagnosed. It is also a very "popular" diagnosis right now, just as ADD/ADHD and Asperger's Syndrome have been.
It is very important the child be diagnosed by someone who specializes in Pediatric Psychiatry, because they are the most familiar with the symptoms, as they manifest somewhat differently in children than adults. Ped Psychs are also more up-to-date about meds and therapies that are effective in peds.
In initial diagnosis, try to avoid folks who specialize in one particular diagnosis or screening. They are great to work with after diagnosis, but can be somewhat partial to their area in diagnosing. In other words, try to avoid someone who deals exclusively with BPD peds in initial diagnosis, as they tend to hone in on the symptoms that marry up with that diagnosis, and disregard other factors that can occur concurrently.
Also, let your friend know that there are a lot of other disorders that can overlap BPD, and to push for a complete diagnosis. Frequently there are other issues in play, and working on them as a whole rather than pieces seems to be better for the child's prognosis.
You can PM me if you have more questions or need more info.