Well, sometimes you Spare and sometimes you don't.

Frisk isn't keen on being grabbed right now, and even less keen on the giant wave aimed at Sans. They try to twist away and, because that often doesn't work, also kick out at her shin. Lapis isn't a monster, so they don't have to worry as much about being gentle with her. It's for the best - a mistake like that would probably have ended with a reset back home.

Maybe they should try calling for help again? That seems like it might freak Sans out.