Do you have to 'friend' someone on facebook to read messages sent?

I have myspace and my son has friended me and my daughter hasn’t. I haven’t asked them to, and I know they both have fb too. I know my daughter’s father (my ex) reads her emails, from me, I don’t know if he reads all of them or just mine. I want to have a word with her privately without him reading it, so I messaged her on fb. I don’t want her to feel like she has to friend me or anything, I just want her to read my message. So, my question is, can she see the message and know what I said without ‘friending’ me?