Wait a second now - if your son DID say this to his mother, she does have a right to be alarmed. Did you ask your son if that's what he said? And if so, what he was talking about? I'd start there. If he says he never said such a thing, or if he considered something like shopping for someone's b-day present a "secret", then you should explain that to her.
If none of the aforementioned is true, then yeah - she's trying to yank your chain. Just know that if your son told you what he'd done with your ex, and mentioned a secret, you might try to get to the bottom of that, too. Just sayin'.
Kel