When you bind a fixture to an instance using physics_fixture_bind this returns an "id" for the bound fixture. You can use this id to get the friction value of the bound fixture (not the "base" fixture) at any time using this function.
|fixture||the id of the bound fixture|
var fric = physics_get_friction(fix_id);
physics_set_friction(fix_id, fric + 0.1);
The code above gets the current friction value for the bound physics properties of the instance and then sets them to a different value.