The situation you've described has an important factor to consider: She's the center of attention in a room full of her peers (or worse: superiors).
Imagine yourself at the front of a room, giving a presentation to a group of people you expect are looking for any way to attack you personally and/or ruin your career, barely hiding the fact you're one of those people for whom public speaking is a fear worse than death, when you suddenly feel your pants split open in back.
How well could you hide your mortification? Could you finish the presentation gracefully? Would it still be a success? Unless you happen to be an amazingly confident speaker, chances are you'd be so flustered and spend so long trying to recover from the embarrassment that the whole thing would be a huge failure.
The cure may be worse than the disease.
Weigh the situation first:
- Is it likely to happen (again)?
- Is/was the exposure significant?
- Did/will a lot of people see?
- Will it significantly harm her odds of success if not addressed immediately?
An intervention is probably unnecessary
Unless the situation is pretty serious, it's best to just ignore it and focus on the presentation instead. Treat it like any other presentation mishap and try to be as forgiving as possible. Even if it is serious, it might be best to forgive anyway unless you're an unusually tactful person or know them personally.
After the presentation, wait until she's finished fielding questions from other attendees, then briefly mention it in private.
Hey, I just wanted to let you know your shirt is a bit revealing when you turn quickly, in case that was something you didn't intend.
While the situation will still be embarrassing, it'll lack the extra potency of occurring in front of a crowd.
She'll likely just thank you for letting her know. If so, just say something like "No problem" and change the subject. Ask a follow up question or compliment her on part of the presentation you liked to show her that you've moved on and forgotten about it. People prefer their embarrassing situations forgotten, so she'll likely be relieved that you've essentially already done so and be happy to change the subject.
If she gets defensive instead, apologize and promise not to bring it up again.
Okay, I understand. I'm sorry I upset you. I won't mention it again.
Then finish up the conversation and leave.
If an intervention is deemed necessary
Use extreme caution and only do this as a last resort. You've effectively decided (for her, without her input) that risking her public humiliation and the ruination of her presentation is a good idea. You'd better have an exceptionally good reason to do so.
First, briefly glance at your phone or laptop if available and act somewhat surprised or anxious. Approach her briskly (a fast walk, maybe slow jog). She'll most likely notice your approach. Try to approach from the direction she's looking to ensure this, if possible. Be discreet, if possible. Once near her, very quietly inform her what's wrong. An appropriate volume is just enough for her to hear, and ideally quiet enough that nobody else can. Be brief.
Your shirt isn't covering you when you turn quickly.
Give her a short moment to respond if she wants to. If you get an angry or defensive response, apologize quickly:
Okay, sorry I mentioned it.
Then go back to your seat and don't talk any more. You can try to apologize again later in private if you're feeling brave, but it's probably best to just leave her be.
If you get a neutral or positive response though, go back to your seat and pretend like the interruption never happened. If anyone asks, deflect with a vague comment:
Something important came up.
The glance at your phone or laptop earlier should help defend the position if anybody was paying attention to you.
If she excuses herself to fix the problem and it seems necessary, apologize to the group and/or reassure them she'll be right back. When she gets back, ask something cryptic that she can quickly answer yes to, like
Is it all right now?
Act relieved at her response and apologize.
Okay, good. Sorry about that.
Support any excuse she provides, if necessary.
It's generally hard to resume a presentation gracefully after this kind of interruption, so be ready to help her there too. If she doesn't just pick up right where she left off, don't leave her stranded. Ask her an easy question to help her get back on track. Something she already said recently might work well.
You may have said this already, but why are there so many samples near the bottom left of this scatter plot in particular?
Then try to relate it to what she was saying just before you interrupted her so she can move back into her planned presentation.
Oh, so is that why the process keeps failing so often?
After the presentation, approach her as part of the after-presentation group and compliment her on something she did well. Be specific: Mention how well informed she was, thank her for being so good about answering your questions, mention something surprising or useful she said, tell her she was an engaging speaker, etc.
I just had to say I'm impressed you already found out the samples were biased. I thought for sure it'd take weeks to do that. How did you find out so quickly?
Try to do it while other people are still there, if possible, to reduce feelings of being threatened and hopefully induce others to say nice things too. It should also discourage her from bringing up the embarrassing event again, and you should act as much as possible like you've already forgotten. If all goes well, she'll assume everyone forgot, remember it as a minor hiccup in a generally well-received presentation, and go home happy.
And intervention or not
Never, ever mention it again.