Again, conversion shouldn't be an issue because tape measures have both inches and centimeters.
I'm able to read, please don't restate things at me when I'm trying to help, I find it very bad netiquette. Thank you.
What I was alluding to is that it doesn't matter if the OP only gets one set of measurements in inches or cms as it can easily be converted afterwards.
Also you've said "most" tape measures yourself, it is a possibility that she'll only get measurements in one set of units BUT as I've stated, she doesn't need to worry about this.
As someone who can sew and does make their own skirts/dresses I only like to work in one set of units (inches in my case, I don't know why) and my tape measure has the CMs on the reverse so I wouldn't take them down if I was measuring someone.
I would also disagree strongly again with the advice for the OP to do it herself, especially if she has no sewing experience, she'll likely measure in the wrong places and the dress wont fit properly, a bridesmaid dress is not a knitted sweater