you need an exaust fan/vent

[MistressEll]

at least 100 cfm possibly stronger depending on the size of your bathroom, vented to the outside, and used whenever moisture producting activities take place and continuing to be ran until the moisture has been vented completely. Opening a window might have helped but is rarely adequate especially for the long hot showers modern teenagers are known for.

Until the walls have dried out and kept dry, any surface repair wouldn’t work.