I was already planning on having Muffet be in the Water leading at least one person back, and her power is literal spiritual tethers, so... possibly she meets up with them while they're both undergoing the Trial, and helps by adding some extra oomph to their connection?

Like, they haven't know the poor people for very long, but Keith's willingness to save them is enough to make the connection, and Muffet will use her purple magic to help strengthen it to make sure it's enough.

After all, they just said you couldn't look back, nothing wrong with helping a guy out if you happen to run into him as long as you don't turn around.