It IS unacceptable to police what people WEAR!
That’s not actually true though.
There are all sorts of situations in daily life where there are specific expectations of dress codes.
It’s disingenuous to suggest that it’s fine to turn up at a wedding in jeans or whatever.
I think you’ve hard a hard time on this thread OP.
We’ve had threads on MN about how inappropriate it is for a guest to wear a red dress to a wedding (ffs) because it steals attention from the bride. 
If it had been a MIL who dyed her hair pink MN would have exploded in indignation.
I think there have been some pretty unkind comments to the OP on this thread. It doesn’t show MN in a good light at all. 
IRL I think most women I know would have raised an eyebrow about a friend choosing such a radical change a week before being a bridesmaid.
In your position I’d say no more about it to your friend OP. I would imagine she won’t look great but that’s her choice.
I’d also quietly speak to the photographer and see what they suggest. A previous poster suggested getting a selection of pictures in black and white - that seems like a good solution.
Don’t mention the photos to your BM at all, she doesn’t need to know about any changes or accommodations you are making.
Personally I’d make sure there was one of her in her full glory and give it to her as a gift.