I think it's odd that you think you know more about this person's relationship than they do. You don't know what it's like behind the scenes. It's not up to you to fix the relationship either. They have to work it out together, and if they don't, that's their choice.

Also this article doesn't explain how "trust" is "toxic" in monogamy. All it does is try to say that monogamy itself is toxic, or the expectation that people will only ever be attracted to one person is toxic.

But that has nothing to do with trust. Most monogamous people - they feel attraction for others but they don't act on it. That's the difference. Or if they want to act on it, they talk to their partners about it ahead of time, and it becomes ethical polyamory.

Trust is expecting your partner to honour their commitment that they made. It's expecting your partner to be honest with you.

That's totally reasonable.