Yes, precisely.
And if they deny it, then just tell her to keep her eye out for it. Either way you win.
If she DOES believe you, then they'll stop.
If she DOESN'T believe you, then she'll keep an eye on them, and they'll still stop.
Problem solved. You're welcome.
“…Judge not what a man has done, but judge what he could have done if he was a different bloke altogether. For art thou a leper? And a leper can changeth his spots…” --Rudy Wade, Misfits (Series 4, Episode 8)