Knowing her, I'm sure she didn't mean to flash people.
I sent an email that simply said, "don't bend over in that dress."
She emailed back, and I think she wasn't offended or anything.
I'm curious about why someone thought it was a bad idea to say anything in a trackable way.
I picked email because it was the ONLY discreet way. We sit in cubicles, and people would be able to hear me if I called, and we'd have to talk in low tones her desk, which would actually be noticeable.
While that wasn't my comment, I can think of a couple of good reasons to keep that sort of thing off of email:
1. If email is monitored and someone from HR saw it, your coworker could possibly get reprimanded by HR/get something on her record. (Obviously, this depends on the office environment. If you're very laid-back, chances are she'd just get a reminder not to wear that dress.) Because it's clear that you don't want to get her in trouble, inadvertently letting HR know might be a problem.
2. It's tough to say "you flash people when you bend over in that dress" without admitting that you were looking at, well, what they were flashing. It's possible that your comment could be misconstrued and make her uncomfortable. An email could wind up getting you in trouble if it wasn't worded properly.
It sounds like you handled it well. I'm glad you clued her in. There are a lot of things that we (general) are expected to know about professionalism that aren't necessarily obvious until we've been in the environment.