Mamoru at least used a panacea on Rodias to try and undo the petrification, though that was mostly a gamble since Mamoru has no idea what the nature of the status effect is. In my mind Panaceas are good against things like magical poisons and venoms, but a Medusa’s petrification May be more of a curse effect in which case Panacea won’t work. But she tried to help Rodias save himself! Asi she’s gunna die so might as well use what is left of her life force to slow down Morgan. 

Also honestly while Mamoru [i]heard[/i] Rodias’s orders, she elected to ignore it specifically because she thought it was a bad idea for him to meet this person alone.