Not sure what to do (venting)

The message is from my SO daughters mother.  The SO wasn't sure if his son told the daughter so the SO asked the daughters mother. 
My SO has a past hx of substance abuse (when the daughter was conceived during a one night stand he was high) he's been clean for years now. The daughters mother uses the child like a game piece. 
Our child will be his 3rd (his first was born when he was 15 and his mother adopted the child) 
The daughters mother apologized... Then asked for my SO to fork over $200 for half of a birthday party he's not invited to (and according to the daughters mother, im not allowed to be around the daughter) 

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