The next time your friend expresses concerns about her daughter's lack of speech, I would strongly encourage her to contact the school district/First Steps/Early Intervention for an evaluation. Yes, many kids have normal "late talker" kids... I have several friends who expressed concerns over their child's lack of speech around the age of 2, had their child evaluated by the school district and we told there was no reason for concern. However, my 2 1/2 year old son suffers from a serious speech disorder that was identified when he was 16 months old (he's been in First Steps since 8 months due to other issues) and any speech therapist will tell you the sooner you seek intervention, the better off your child will be. My son's speech disorder basically affects his expressive language - what he is able to communicate to others. His receptive language - what he understands - is age appropriate, but because he understands what is being said and knows what he wants to say, he has increased frustration because he can't get out what he wants to say. Could your friends daughter's "high pitch scream" be her extreme frustration that nobody understands her???
In my experience, even the best pediatrician isn't clued into severe speech disorders and won't get too concerned until after age 3 when a child has missed critical speech therapy time. We have an awesome pediatrician and when I told her I was checking into early intervention, she didn't feel it was necessary and that he would be "caught up" by the time he was 2 or 3. I trusted my instincts and sought the evaluations (they are free through the state, so all it costs is time). I am so grateful that I did because my son's speech is much farther along than other kids his age with the same disorder (but were diagnosed/started therapy much later) and he will more than likely completely overcome these issues, although we still have several more years of speech therapy ahead of us.
Feel free to ask any additional questions. I would also look into the speech disorder, Apraxia. I believe on the www.apraxia-kids.org website, you can find the symptoms/signs of Apraxia. Apraxia is a neurologically based speech disorder and differs from a speech delay. A speech delay is something that even without therapeutic intervention, a child will eventually catch up with their peers. A speech disorder is something that a child will not overcome without intense speech therapy.
Best of luck!!