Hi there,
I've installed it and it works. I have some suggestions for future versions. I could do them all by myself but I don't have the time at the moment
- Possibility to send the pm from a defined user account or from the user who added the recipient. For example: I use a "Bot" to store some external information into the forum. He could also be used to send the pm. Maybe some others would like that, too.
- After a quick look on your code, I found there is no check if the recipient already has added the sender to his buddy list. If that happens maybe another phrase should be chosen without a sentence like "If you would like to be friends, please add me to yours", i.e. just "I've added you to my buddy-list" or "User $1 has added you to his buddy-list". Got my idea?

If more gets into my mind, I'll post it.