Guess why they do it? It's hardly rocket science when people do something that they know upsets you, right?
How about NOT responding at all when it happens? Guaranteed she'll get the information from SD, all the information! It's NOT SD's fault that she imparts information, right? BM's have a way of manipulating kids into spilling the beans.
How about just a momentary 'Oh!' and switch your attention right back to where it was. Later you can pass a comment to DH, after SD is out of earshot.
I'd say 'That text really could have waited until the end of the movie, you know.' It lets him know that it was innapropriate but not forbidden. That's the thing too, you tell someone not to do stuff and what's the first thing they go and do? Yep!
Gradually say stuff that is not going to condemn him but make her look bad, like 'Is she really so insecure that she has to keep calling?' or 'Doesn't she have anything better to do?' Gradually make the words stronger but not in a condemning way towards him and he'll get the picture. He'd want to blame her too so he'll go along with you there if that's the path of least resistance. BM holds the ace card and DH is just randomly trying to pick a trump card here. It's NOT entirely his fault.
He is with you, not her. He might want to make his future with you and the worst thing ever is to be negative about his history, we ALL have history. Anyone who has had kids has to walk the fine line with the other parent, if the parent is in the picture, an SO really does need to accept that BM will be in the picture at least until the child is 18. THE worst thing you can do is let her get a sniff of any ill feeling or insecurity you might have...she'll pounce on it and use it to the MAX!! Right? Next thing you know SD will be PASing because BM knows she can get to you...nip this one in the bud and be luckier than I was...I urge you! I endured three years of hell!
Develop a coping strategy, imagine her in a compromising situation and smile to yourself!
I told DH that what I felt was grief, that he and I did not have the 'firsts' together and that's possibly why I resent BM a bit. He actually said he'd thought about that too. I also told him that the 'old familiarity' made them look more connected and I was on the sidelines if she happened to be where we were, I said that she'd use that to further her own purpose and she knew all the buttons to push to get what she wanted. DH denied it at first and then asked me to point it out when I noticed it so I did! He became aware and he focused his attention on me, I actually...dont judge me...asked DH to lean in and listen and all I said in a low tone was 'sweet nothings sweet nothings' and he looked me in the eye, smiled and chuckled. Genuine smiles and laughter eases uncomfortable situations. BM was LIVID! Sorry 'n' all but he's my husband! I said don't judge me! LOL