Multithreaded programming in C (on Linux) is easy++. On Windows, it's painful.
Incidentally, if his goal is to program a Gaim plugin, I don't recommend learning C++. It's better to learn C anyway.
A book that I'd recommend is Programming Pearls. I forget the author, but can dig it up if necessary. It doesn't teach you the language, but it does teach you some nice tricks about how to program well.