The Kate Hudson character is an ignorant idiot.
SPOILERS.
When you're ignorant you can't see anything else except for the limits that you've put in your sights. Kate Hudson's character was so self-absorbed that she didn't notice what was going on around her in her own life among her fiance and her best friend.
Get out of your constant daydream and you will notice s*** like that! Especially when you see the people on a regular basis, you will see the chemistry or you will a least see a sign or signal that shows that at least one of them is attracted to the other.