My H's kids' mom has many BPD traits, and is married to a guy with many NPD traits. ~8ish years ago H had to get a lawyer to protect his parenting time with the kids. Mom did escalate mostly through roping the kids (then 8-ish and 10-ish) in to adult issues and emotionally manipulating them to be on "her" side. Lots of blaming emails to H, lots of continued subtle attempts to interfere with his parenting time.
A couple months ago, we had to get CPS involved due to things the kids had told us about their mom's house and their stepdad's behavior. I expected significant retaliation (I made the call) but got nothing. Mom did blame our oldest (SD18) significantly for the situation, but has not interacted with me in any way since then, not even blaming emails/texts. She has not tried to interfere with H's parenting time or with us taking our youngest (SD16) to school.
So, response can vary after legal involvement, and it isn't always predictable. Or, I should say, it's predictably unpredictable. I couldn't rule out that Mom and Stepdad might impulsively try to contact my workplace or show up at my work (they didn't, after all), so I emailed HR ahead of time letting them know that we were in an active CPS investigation and to be aware that they may receive contact from CPS or from the kids' mom/stepdad. No matter what your other child ends up choosing (RO or not), giving workplaces a
proactive heads up is a good protective move.
RO's are one strategy; there are others to try as well. It can depend on the content of the messages and to whom the messages are directed. Blocking and phone/email change, and/or filtering emails from D30 into an unread folder or trash, would be a reasonable first step.
It is important to know that while there can be benefits to RO's, there can be significant drawbacks as well. Taking out an RO means you have to be committed to contacting law enforcement if/when it's breached, so it wouldn't be just one escalation (getting the RO), it might mean multiple escalating instances (calling LEO's
every time a new message is received or there's breaching contact, not just "when I feel up for it").
If your child hasn't yet read
The Gift of Fear, by Gavin De Becker yet, that's a good place to start for straight talk about RO's. They definitely aren't one-size-fits-all.
Stepping back a bit, the other thing to consider is that at the end of the day, your child is making the choice... not you (I'm assuming your non-BPD child is also an adult, not a minor). Finding a way to "read the room" on if your child wants your advice is important. As the parent of an adult child, your role has shifted over the years, away from the hand-holding that is so loving and necessary when the kids are young, to more of a "we'll be beside you as you choose your path" role. More freedom... definitely more fear. I get it.
Has your non-BPD child asked you for your thoughts on the situation?