My friend is flying with me and my Mom to America, but our Insurance doesn't cover her because she is not family.

& She has been refused Travel Insurance from the post office and online because she is under 18.

So is there a way we can find out how my friend can get it?

Not sure what your talking about here, you can buy personal insurance for the flight at the airport, if you mean insurance for medical overseas whatever she has now should cover her there she needs to talk to her insurance agent. Or have her call a travel agent about it they may know of an agency that covers people for the duration of a trip.