Could you provide some sources where you have learned about meeting people on the "mental plane" (distinct from the "astral plane"), and "frequencies"? What you are describing is not something I've come across in my studies of the occult, at least not in those terms.
Perhaps what you are looking for are actually lucid dreaming techniques.
There is a lot of material available on the topic of lucid dreaming both in print and on the internet. One document on lucid dreaming that might be of some help to you is Lars' Lucid Dreaming FAQ which is available at
http://www.faqs.org/faqs/dreams-faq/lucid-dreaming-faq/
I suspect that what you are looking for is actually just a particular form of lucid dream. I would be suspicious though of any claims that what you are dreaming is authentic communication with another person (your friend in this case) rather than just an induced dream that happens to include your friend as one of the dream characters.
Anyone who could prove and reproduce actual communication between two dreamers should submit themselves to the James Randi Paranormal Challenge as they have a million dollars US to hand over to anyone who can do things like this. The challenge website is at
http://www.randi.org/research/index.html