I often see people asking the question: "Should I write this about [person I know]? How do I know what would be okay to write about, and what not?"
The person they know may be anyone close to them, in this case.
I think the following is a good answer, and one I would try to go by (although now that I think about it, I don't know if I have kept to this in the past, despite not having written a lot of blog posts yet).
If you think that the person you know wouldn't like you writing it, don't write it. If you think that they would not like reading this, by you, about themselves, don't write it.
Now, of course, this is pretty logical advice, and I'm not the one who made it up. It's often given as an answer to the question at the start of this post. And then the person who asked that question sometimes says: "But [person I know] will never read this anyway (for whatever reason)". Implying that it's then okay, or at least not so bad, to write it anyway. If they say this, they missed the point. It's not about whether they will ever read and understand it, it's about if they could read and understand it, would they approve of you writing it? If it's not right when they read it, it's still not right if they don't read it.
Compare it to stealing (may be too harsh an analogy for some people, but it works): It's not only wrong when you get caught. Not getting caught doesn't mean that it wasn't still a wrong thing to do.
So, I at least will try to imagine what people close to me might think of what I write about them, from now on, and refrain from writing it if I think they would not like me writing it and if they would not like reading it.