I’m curious to who you think is wrong here. For reference I’m neither person in the scenario but I am friends with both and feeling quite in the middle of the drama! 
Friend A - had a wedding abroad, beautiful destination. Invited friends and family but understood if others couldn’t make it due to the cost etc. most people ended up coming and all stayed a week or so.
Friend B - came to friend A’s wedding with her boyfriend of 3 years. enjoyed the wedding together and then continued the remaining 3 days on the holiday with the boyfriend. boyfriend proposes on the last night of the holiday, friend says yes, very happy etc
Friend A is angry that it’s taken away the ‘spotlight’, so to speak, and that they’ve got engaged just a few days after the wedding.
Friend B thinks friend A is being unreasonable as this was also their once a year holiday, they spent a lot of money to come to the wedding and the boyfriend was planning to propose on whatever holiday they went on this year anyway.
So what do you think? Is Friend A being unreasonable for being upset?
YABU - Friend A shouldn’t be upset, Friend B is right
YANBU - Friend A is right to be upset about the situation