@SerPounce, the non plugin way to do this is through url parameters. Have your Filter On sub-menu navigate to the page your RG is on (this will be the same page as the RE when it’s actually used) and pass a parameter of ‘filter’ with the value of ‘on’.
Then simply access the filter state with a Get data from URL action. Works perfectly.
The other option is to use one of the global variable plugins available as they will pass data across reusables and pages.
Edit to add: You can pass things (i.e. your user) as a parameter as well by sending the unique id.