So, every time my 'best friend' is over and visits Lachlan, if she's holding him she sticks her…

So, every time my 'best friend' is over and visits Lachlan, if she's holding him she sticks her finger in his mouth so he can suck on it. Doesn't wash her hands first, doesn't ask me if it's okay and even does it if he's not crying. It pisses me off to no end and I've told her not to but she keeps doing it. Is it acceptable to cuss her out if she does it again?