Any kind of autism can improve with therapy and support. It depends on the child, it depends on how well you can connect with the child and it depends on how early you start. Basically, you give a try to whatever works, the sooner the better. Getting in early to work with a child may do little or it may do a lot. It's likely, however, to do far more than nothing, which is what can happen when you don't diagnose or you fumble in the dark (as we did).
difficult child 3 did not have regressive autism - he simply didn't develop communication until he 'connected' with reading. It is a communication rather than language disorder. There is a big difference.
difficult child 1 did not have any language delay, but he does have some communication issues although I think they primarily revolve around the problems he has in mentally multi-tasking. It's a memory thing.
We have known kids who developed normally and ten regressed. it's much rarer. Do encourage your friend to check this out ASAP. Sooner, if possible.
And if she's panicking about it and in denial, get her to read "Son Rise" (not sure of the author). It's about the author's son who was diagnosed with regressive autism. While it's not necessarily representative (and in my opinion is a bit waffly at times) it gives a lot of hope as well as some ideas to try while waiting for more specific support and information.
Marg