If you want to send a text to create this boundary thats fine.Since you have been being a friend. I would start to text off so it don't sound like your were bothered by you'll last meetup.
I would say, "Thanks for dropping the dog off, please let me know before you drop the dog off next time."
If she ask why, you tell her it will make you feel comfortable knowing".
Don't let her draw you into an argument or discussion over this subject.
If she ask, what if I don't(she's becoming disrespectful) you tell her you will change the locks. (This is her consquence for crossing your boundary).
If you keep the convo, like above you stay in control, if you let her draw you into an argument or discussion on the M or R, she will be in control. Shes knows you are vurnable there.
If you do get drawn in. Get to your senses and tell her, you don't want to talk about the R or M, while she is involved with OM.
If she ask, is this because of the other day. Tell her the truth. Be honest about your feelings and then end the conversation. This will be hard for you to do, because you are going to want to know how she feels. (She don't know how she feels, so you will never get an accurate answer, while she is WW).
If the convo go down like this, she will probe and test you to see where you are at in relation to her. Don't let her.
Since you are the friend and husband as you have explain, you pulling away will be a lost to her. At this point she must make a decision. This is what you ultimately want, for her to make a decision. You can't control her decision, and we hope it is you.
Stay strong and continue to post.
M:37 W:37 T:11 M:10 S17, S13, S10, S4 BD:06/28/17 OM confirmed 07/20/17 Recon the M 10/29/17 Working hard:2gether
Onward and forward
This process is not a sprint it's a marathon! Patience, Patience, Patience.