Me and one of my close friends found ourselves in the same position! She was engaged a few months before me, she booked her wedding date for sometime in July and we booked ours for May. She became really off with me, even tho I was her bridesmaid and vice Versa, she kept the colour details from me, wouldn't talk about any of her wedding plans, even down to food ideas even tho our weddings were total opposite ends of the scale, we were having a very old fashioned DIY wedding, family church, village hall, 2 course buffet and family made cakes for pudding, she had a very popular expensive wedding venue, 3 course sit down meal, all the bells and whistles.
She refused to pay out for my hen do even tho I would have been at hers if i had to sell a kidney, and when it all came out she said it was because I'd booked my wedding for before hers even tho she was engaged first. Once I'd explained the reasons for May ( our anniversary) she kind of felt abit silly and we've managed to move on from it and are still the bestest of friends but even now looking back it makes me so sad to think she felt the need to compare and worry when really if your a true friend it shouldn't matter!
On comparing weddings, I kind of relate to this too, my cousin who I'm quite close to got married 3 years after ours, however she had same family church, same venue, same cake maker, same colour scheme, even the same flowers but the icing on the cake was that she booked my makeup artist and same hairdresser. I took this as a great compliment that she wanted to basically replica my wedding day and altho I worried it would be compared people just enjoyed the day and I think if you just rise above it and be there for your friend, both days will be equally beautiful and unique in their own ways!