I would not give much thought to what some fan said she hear especially bc all people that were there said that they were kept far away and could not hear one line, but sudden this girl was close to hear lines but not all of them, i mean if she was closer than other fans to hear the lines she had to heard them all, not just part of them and is not the first time some fan makes things to get attention.
YVRShoots also overheard the dialogue, so no I don’t think she’s making stuff up:
http://yvrshoots.com/2016/01/shoot-once-upon-a-time-shipper-alert-hades-greg-germann-zelena-rebecca-mader-in-the-underworld.html#.VpFVvvkrKUl
Something to keep in mind is that the other fans who visit set do also overhear stuff or find out about certain spoilers, but they choose not to spoil them online. Kat does the whole “I know stuff, but sorry can’t spoil it” thing all the time. For example, she was one of the fans who watched them film Hook’s death, but wouldn’t say anything about it until the episode aired.