Sucks for whoever fell through. Sans is going to cast web with the ropes as anchor points and try to get down that way. He'll actually start with a web somewhat higher than he is right now and misty step up to it so that if it gives way under his weight he can misty step back to the cliff. Assuming that doesn't happen, he'll repeat the process, casting web and then misty stepping down, to see if he can reach the bottom.