I have my boyfriend's mum as a friend on Facebook?

[Chatty Cathy]

It's totally fine to have her as a friend but....

Yes, be a little cautious when you post something, such as a picture or a status that can be considered "inappropriate"

An alternative in case you don't really want to change what you post, is that you can create a list and add people in that list, and name it, so, when you post something, just click on the setting thingy on the post, and there should be an option to hide from a certain list, single person, or just simply hide from timeline.