It's been, at least loosely implied in canon that Rarity's accent is fake. And I saw at least one person say that they confirmed it in the Season 1 commentary.
Fanfics have dealt with this, usually they're surprised but not too terribly bothered.
I don't know how I'd react if I was in Rarity's friend's shoes. I wouldn't be horrified, but I would probably, at least, be a bit uncomfortable. I mean, they've been besties for years, and Rarity has never once dropped the accent. There is no indication they know. There is no indication Rarity told them offscreen. I can see Pinkie Pie not being really all that concerned, but she could probably help Rarity understand why the others might be a bit put off. Ugh. Even as I write this I know people are saying, "BellChime, it's not like she pulled a 'Season 4 Finale Discord', why would any pony be put off'. But that's the funny thing about this possible problem: It's definitely not a big thing (like season 4 finale discord) but it's not exactly a tiny thing, either.) It's like in "Secrets and Pies"-that does an excellent job of showing how a small deception, if you practice it long enough and thoroughly enough, can really, really hurt somebody's feelings. Anyway, I think it would be interesting to explore possible negative reactions...not saying there has to be this dramatic "HOW COULD YOU I THOUGHT WE WERE FRIENDS" blowup, or anything.

7118143
I’m not sure the other ponies would mind, even if Rarity outright told them. There is such a thing as cultivating an accent, which I would think is what Rarity does.

I do not imagine it would be a big deal. It’s a lovely voice.

7118143
Honestly, it seems no more relevant to me than her doing up her hair. It's part of the image she tries to project and doesn't really matter much at the end of the day. Also, considering they all know her family (especially Sweetie Belle) and are fully aware they don't have that accent it seems obvious enough that I'd expect them all to know already.

That said, if you really wanted to do something with it you could probably get away with using AJ for that since she's dumb enough she might not have figured it out essentially immediately and tends to but heads with Rarity, but even then it seems more likely she'd just use it as one more piece of ammunition for their spats.