as opposed to a stranger.

I mean nowadays, you often have quite a big choice when going for a sperm donor at a clinic. Yes, you won't know them as a person, but the donors are checked thoroughly, and you often know them "better" than some of your friends.

When they're old enough, we can explain why me and the kid's mom are not together.

Sounds good on paper, but often it doesn't turn out that way in reality. Before moving forward, you and your friend (at first) will have to sit down together and hash out all the smaller details. What will your role be? What if you're not okay with the way she raises her kid? What if you disagree about when the kid is old enough to know about the sperm donation? What happens in the future if there's another potential parent/SO in the picture? What if the kid and the mum have issues and the kid reaches out to you? etc. etc.