Ultimatums and telling someone what to do are about contolling the other person.
Boundaries are about setting limits on how you allow people to treat you.
The only way to get the difference is to try to start practicing it, though some reading, like the book I mentioned to Nikki, can be helpful.
Nikki needs to set and enforce her own boundaries and own the consequences of doing so. Nikki is a big girl. Her H has repeatedly demonstrated that he will continue contact with PW right now. Nikki either needs to be OK with that or distance herself enough from him that she does not take it as a personal injury.
She is an adult, not a helpless victim.
Right now, she keeps throwing herself in front of H's car and crying when he runs over her. She asks him to be more careful, he keeps running over her. (Rinse and repeat.) Time to quit throwing herself in front of the car. She is an adult, she can figure out what to do instead.
