In your shoes I'd phone her & have a bit of a heart to heart.
I love my husband to bits & think that if push came to shove I would fight for him if he ever became...distracted by someone.
I would be careful to be polite & have an honest woman to woman chat, no accusations, just called as I noticed the volume of texts which has made me worry & want to know what was going on....blah blahblah
Unless she is a complete psychopath she will be:
a\ intimidated that you had to balls to call her (I mean on her behaviour...who in their right mind exchanges so many texts with a married man)
b\ You are being territorial about you family....my DH...my domain...I have every right to know who you are
c\ Demonstrating that this goes no further without a fight
d\ You are also showing that you are a person...whose feelings she is damaging by her behaviour
I know someone who did very similar & managed to see the other woman off.