Yikes! I reckon the key thing is to lower stress for you, and maintain the friendship. I think you need to have a conversation with her not about the dress, but about friendship and stress. Tell her that your main goal is to avoid stress and for you two as friends to respect each other on what should be a great day. Tell her what that looks like for you (a compromise maybe, involving straps but not length?) and ask her what that would look like for her. She may not realise the impact she's having on your stress levels, and once she does, she might be more up for a compromise.
FWIW I've been a bridesmaid twice, and for one (sole bridesmaid), my best friend paid for me to go out and buy any beautiful pound dress I liked up to a certain price -- we have the same aesthetic and I consulted her before purchase. We looked amazing together, and I wore the dress heaps later and still have it, though it's been bashed by having too good a time too many times. This is the optimal scenario. For the other, I was in a team of three, and I was the tall one out and the others were short, and I compromised on a dress that she paid for that looked basically ok but not amazing on me. As the bridesmaid, I had considerable 'appearance stress' that day. I crash dieted for a month, which I never do, put all all this bloody fake tan and wore annoying heels. It was fine, but it would have been better if I had a different dress I felt good in, and the bride would probably have been up for a bit of non matching.
I say all this in order to I recommend the former route, where everyone feels good in their bodies in their dress. Less stress all round and more fun. That said, if your friend just wants to optimise her dress or look unique or something facetious, rather than having serious misgivings about how she'd look, then that's not really valid.
TLDR: have the meta-conversation. Better do some work now. LOL my own wedding is next week and I'm one of those registry office smug fucks cycling down in a high street frock so GRAIN OF SALT.