My mil refers to my bil's bff (wierd relationship - they were engaged, living together, broke up and still friends and living together) as "aunt" to my kids and I can't stand it because I don't know her that well and my kids rarely see her or their uncle.
On the other hand, we refer to our bffs as uncle and aunt sometimes. Not all the time. The only family we have living in our area is my dh's step brother, whom we never speak to. Our bff's are more of aunt and uncle to my kids than their real aunts and uncles. They live 30 minutes away and have come to my house to take the kids out when I've been sick, they babysit for us sometimes, every time they go out of town, they bring our kids a present, they go to my son's soccer games, they are also our additional emergency contact for the kids at school, etc.