It's not a good idea to send the ow a message but you can give her one indirectly by telling your deceitful twunt of an h that you will be petitioning for divorce of the ground of his adultery and will be naming the ow as co-respondent.
Once he reports that back to her and she finds out that being named as co-respondent could result in her having to pay part of the court costs, I reckon she'll get the message that you're not to be messed with. 
However, you've said that he's never even met her "in person before". If they've yet to meet scrub the above and we'll work out another way for her to get the message without it coming directly from you.
How old is your dd and how often does he come to 'visit'? If these visits involve him coming into your home and staying in it in order to see dd, this has to stop and in future he can collect her at an appointed time, take her to the park/soft play or whatever is age appropriate, and return her at whatever time is agreed between you.
When/if he has his own place, dd can spend every other weekend with him but you must make it clear to him that it won't be in dd's best interests to meet the ow until she has come to terms with him leaving and that could take 6 months or more. How has she reacted to her df having left the family home?
There's no absolutely no reason whatsoever why you should stay quiet about him leaving you and his dd to be with an ow and, for the sake of your own sanity, you need to vent/offload in rl . If his family turns against him, so be it but, as they are presumably all adults, I'm sure they'll be able to prevent any anger or disdain they may feel towards him affecting dd in any way.
At some point you will need to make an appointment with a solicitor who specialises in divorce and family law but as you're still reeling from the shock of your discovery, I suggest you wait until you're feeling more together before doing so, particularly as many lawyers will be on vacation until September.
Do you own your home or are you renting, and has he been contributing to the outgoings and paying maintenance for dd?
for you, honey. You'll get a lot of support here from those who have been, or are going, through the same situation that you've found yourself in through no fault of your own. Be kind to yourself and don't discount getting a short course of anti-depressants from your GP if you feel begin to feel overwhelmed by the abrupt way in which your life has changed beyond recognition from how it was just a few short weeks ago.