I had a similar situation with a good friend recently. I wrote down the three examples that most stood out to me, and prepared some thoughts, then I gave her a call when I knew she was on her own, not with her DH.
I prefaced the conversation with something like “[Friend], I’m about to comment on something that is none of my business, and I hope you’ll take it the way that I mean it: not that I’m meddling, but that I care deeply about you and I’m worried about you. Please hear me out, because I’m feeling nervous about raising this and would like to get through what I have to say before you react, otherwise I know I’ll lose my nerve, hahaha.
It’s about [her DH’s name]. The way that he’s been treating you has raised a few red flags for me. I’ve observed [example 1, example 2, example 3]. In particular [example 3] has been sitting really uneasily with me since I saw it a few days ago, because you deserve so much better. I don’t presume to know what you should do about any of this, but I wanted to raise it with you just in case any part of you was doubting whether these behaviours are normal. From an outsider’s perspective, no, they seem very problematic. They’re controlling, they isolate you, they prevent you from living the life you want to live (you and your DCs), and being honest it feels like they border on abusive, although I know that’s a very loaded word. I think it’s important for you to consider that abuse could be at play.
Now, the LAST thing I want is to make this all one more thing you need to worry about, or to feel like you need to hide anything from me from now on, or ‘stop venting’ about DH. Only you can really know what’s going on in your marriage and whether you’re ok with it, but from the outside there are a few markers that look like you could be in a risky situation, and what I want to make clear is that whatever happens or however you feel, I’m here for you.”
She took it amazingly well. She told me she was starting to talk about these things with her therapist, and that she and her DH were starting couples therapy. She said that she and her DH both agreed the previous week that fundamentally they really love each other and want to stay together, but that they knew there were relationship dynamics that had to change. She thanked me for raising what I saw with her, because it made her feel valued and connected. She texted me again that night to reiterate that, and we’ve talked many times since then about other things - the conversation didn’t imperil our friendship, and I’m very glad I raised it. (And glad that she’s dealing with it in therapy alone and with her DH - I think that in the circumstances, that’s a potentially ok outcome).