Step son wants to take me out for his 18th birthday

I've been in my step son's life since he was 13. He turns 18 Friday and has also been working his first job. He said he wanted to have a mother son lunch and wanted to pay for it now that he makes his own money. He already had a lunch with his dad. I love this kid like my own. He calls me Mama. The issue is I don't want anymore drama with his s mom. She's accused me of trying to turn her kids against her. She blames me on why my step son and her aren't close and haven't been for years. She has harassed me on Facebook and because my other step kids are young they have split custody but with my step son being 17 he's chosen not to see his mom anymore. But I'm forced to deal with her and I know if she finds out he took me off it for a lunch she's gonna be mad