DS3 (12 yo) has a best friend, from Y1, whose DM has been through a bit of a traumatic divorce. The childs mum is now in another relationship with someone who has a bit of money. The mum and I are not friends apart from chatting in the playground etc.
We (as in me, DH, DS's and everyone else) have been as supportive as we can be. We have had DS3's friend overnight several times at short notice because the friends mum has a late "function" to go to with the newDP, he (the child) is round our house most of the days at the weekend as his mum and her newDP have "something to do" - her words, not mine.
He is a lovely kid, I don't object to this as long as it fits in with what I have planned. We have been on days out, shopping, etc etc, he feels like one of the family. But he is not my child, and I am not responsible for him.
This weekend is mine and DH's wedding anniversary and DH's birthday. We have planned to go out, just us 2 on Saturday night for our celebration. DS3 is staying over at a relative's, who is not in a position (and would not want to) look after 2 children (and I wouldn't ask her anyway).
DS3's friend, and his DM, have been told that this weekend is off limits as we are not available.
The friends mother has just rung me and asked if me and DH can change our plans to next weekend, as her and the newDP have tickets to Wimbleden on Sunday, so can DS3's friend stay over with us on Saturday, and stay all day Sunday as they want to stay over the night before until they get home!!!
The friends father is not an option as he has a traumatic brain injury and cannot look after any children at all.
I said "No, we have plans, we did tell you" She said "Oh no, I will miss the tennis"

And she is not a kid, we are both in our 40's.
IANBU am I. 