There is also fetlife.com. It's a Facebook for kinky folks. There are all kinds of groups about various aspects of D/s. There are event listings so your friend and you can go meet other kinky people, if so inclined. Like any other social media, you will find both good information and complete dreck, sometimes dangerous dreck. Keep your critical thinking caps on.
May I ask why you think your friend is submissive? Feel free to ignore me or PM me.
I ask because, well, some folks are submissive in bed, some are submissive in much of their life, some folks are submissive but one would never expect it from how they present themselves publicly. Also, if your friend is a woman, submissive behaviour in bed is how women are stereotypically 'supposed' to be when having sex with a man.
Your instincts may be right on but submissiveness is more intricate than one might imagine.