I thought the ending in which a character transfers away and disappears would disappear with the 90s. Before then, it was actually possible to lose someone because their family had to move away and you would never find out where they went. Today, I think it's actually more difficult not to be found by people you left behind.
I'm a bit surprised that it seems to still be a thing with storytelling. Finding out where she went and keeping in touch should be a pretty easy mountain to scale, especially if mermaids are involved.