Appropriate wedding guest attire

It looks like a bridesmaid dress. What color are your bridesmaids wearing? If it's similar, I'd ask her to choose another color. If not, I'd say its fine. No one is going to mistake her for the bride.

It would bother me a tiny bit. What color is the bridesmaids’ dresses?

Just say no. She's asking you so she knows it's in the grey area.

It wouldn’t bother me as long as my wedding dress was much dressier and as long as the bridesmaid’s dresses are very different from this guest’s dress.

Personally, I would be pissed. “Peach” doesn’t mean anything when it looks like a literal casual wedding dress! That dress in a different color, absolutely. That’s too close to white for me!

Cute dress!!!! Zero concerns.

It looks bridesmaid-ish to me. Is this a good faith question do you think?
I mean...I see lots of posts here were it seems like the guest asks to wear something outrageous just to be a pain. Like “Can I wear overalls har har har?”
So if you think it’s a legitimate case of her not really understanding that it looks a bit too close to a bridesmaid or wedding dress, you can say that. But is the sort to think it’s funny to be transgressive...I’d say “looks kinda like a wedding dress but you do you.” If she wears it she’ll look silly, if she’s asking to get a rise, you won’t give her the reaction she’s looking for.