"On the other hand, I'd be a horrible friend if I were able to help my friend and her DH figure out their way out of this financial pit and I refused to tell them how." I don't think this is true at all. Recovering financial stability from the kind of hole they're in is a complex process that takes a lot of time, effort, and personal discipline. It's not like her cake keeps coming out badly and you're hiding the fact that her baking soda is stale--there's no one magic thing you can tell her that would solve her problems. There is a ton of financial advice out there already and it doesn't sound like she's really interested in following any of it, so it's hard for me to imagine she would suddenly change her mind just because you offered to help.
That said, if you really want to get involved, I think it would be most polite to separate your financial motivation from your friendship motivation. Tell her frankly that her financial situation makes you worry about the safety of your joint investment. She needs to put $X in some kind of untouchable account that will ensure her half of the investment is paid for Y amount of time (or whatever); and after that is done, if she wants, you will happily give her financial advice and/or take her to a financial planner, as a friend. If she's not interested in securing her half of the joint investment, you've already looked into it and will be pulling out using ABC steps on D timeline.
If she chooses the first option, you will rest easy knowing that your investment is secure, and you will be able to give her financial advice truly as a concerned friend. If you just give her financial advice, allegedly as "just" a friend but always with the idea in the back of your mind that your own money could be at stake, I think that's a bit shady, even if you mean well. Take your own stake out of it first. If you're really going to try and help her with her finances you will need to have many frank, perhaps uncomfortable conversations with her, so best to start here.
Though personally, being non-confrontational and not savvy about financial matters, I would just be looking for a way to disentangle myself from her, and not get involved in giving advice.