How do you handle xmas presents with the ex for the kids?

The ex just sent us an email to inform us that she is buying my stepdaughter an acoustic guitar for Christmas (this was after we'd sent her an email telling her that my stepdaughter had told us she wanted one so we would get it for her for Christmas).  My husband is the guitar player, he's played since he was a kid, he's been in bands, it's his "thing" with his kids - to play music with them - like he does with our kids.  He's taught them and he plays with them; she doesn't.  He knows which brand guitars are good and which ones aren't.  He knows neck size for her hands, if the neck is straight, action, wood, he knows what to listen for when he tries them out in the store first, etc.  The ex wouldn't know which end of the guitar to start with! 

We've already bought my stepson an electric guitar, which we didn't tell the ex about.  She'd probably try to stop that one too so she could get him another one first. 

I haven't told my hubby yet about his ex's email.  I wonder what he'll do about it - if he'll still go ahead and get her an acoustic guitar or if he'll find a different gift for her.  If the kids were here more often, I'd say "so what" and keep it here for her to play but they're not here that often. 

This just reminds me of how many times she's come back and said she was going to get them gifts that he had wanted to get for them, how she'd try to get the kids the DVD movie releases as soon as she could to beat us out of doing it first, and how she wouldn't let hubby participate in field trips unless she was there too.  She can never just let him do something for the kids without it being a competition.