In this situation, if you are deciding to cut back on advising her- it's not a spoken boundary. It's an internal decision to change your own behavior. Telling her not to ask you as often is like telling the snake to stop biting, or to humanize this- it's telling her to not feel her feelings- which she's going to do anyway as it's not something she controls.
What she's found is that doing this, often when dysregulated, is a behavior that works for her in some way and your behavior of advising is working to reinforce this dynamic. You are choosing to change your part.
This boundary is as you choose it. It doesn't have to be all or none. It can be but it also can be a choice to reduce it- not completely stop, or to not advise on topics like mental health questions. I think we feel we need to know the answer but sometimes an "I don't know" is an honest response- sometimes we don't know. One boundary that can be firm for you is to not broach the subject that this is your SO's flawed thinking and that they need mental health as you know the outcome of this.
Paying attention to your own feelings can be a clue to when this is a genuine request for something or due to a need for emotional caretaking. Usually we don't feel irked when someone is genuinely asking because they want the answer. It feels different when we are being recruited to emotionally caretake. The request can be similar, and it involves our natural tendency to be nice so it's hard to discern but there was a subtle sense of feeling unease when it was about emotional caretaking.
Sometimes I would just decide to do it, if I was willing and and able to tolerate whatever reaction resulted. Sometimes I didn't know what was going on until there was a reaction. Sometimes advice would feel invalidating to BPD mother and she'd get angry.
This is the kind of boundary that is likely to result in a reaction because, to decrease emotional caretaking behavior disrupts the dynamic but by doing so, it might reset again as they adjust to it if they are able to. It's a risk but it's also a path to changing a dynamic in a more positive way. We can't change another person but we can change our own behavior and then there's a possibility they may or may not adjust to that but to continue in the same way perpetuates it.


