how do I stop someone seeing my comments on Facebook?

[Roz]

I don't want a certain fb friend to see when I comment on other peoples activity. but im happy for them to see my statuses. Also people know when im on FB through chat when im on my iphone or ipad. I want to look offline, yet still be able to reply to any messages. this is not a problem when I use my laptop and I can see an option to turn chat on.

[Cassie]

You can't stop friends from seeing your comments. Who sees your comments is determined by the privacy setting of the post you're commenting on. If it's public, everyone can see your comment. If your friend doesn't have a right to see the original post they won't see your answer either, though.