Re: how to approach
I can understand your concern, and your desire to help her with this, with the least embarrassment possible. Since you share her issues with incontinence, I think you would be a good one to discuss this with her. It is not appropriate to wear diapers exposed, nor is it usual that she has no apparent sense of modesty.
I would be as sensitive as possible, while truthful. Even babies need to have their diapers covered in public places for obvious hygiene reasons. I would have to think that the diapers in the dance club must raise some eyebrows, if not remarks...
The next time she appears with her unde'rs showing, I would take her aside and mention that she is "showing', and ask her what she has to cover up with. If she does not have any backup, explain that she needs to go take care of herself, she needs to change. Her reply should give you an idea of where her head is at about this...Does she realize she is soiled? That is is showing, and could possibly have a leak? Does she carry spares? If she does not have a fresh diaper and clean coverup you will know she is not taking care of herself properly, and not dealing with her situation rationally. Does he have a sister, a girlfriend that you could align with to help her out. If she simply does not care, I would wonder about her mental health too.
At 22 years old, most women would never be caught dead carrying "baggage" for a second. This does sound very unusual...I wish you and your friend the best.