The way you do it, it will execute the method and will try to set the result as callback function
Internally for callbacks class uses call_user_func function:
php.net/manual/en/function.call-use
...
which means to call method of some class, you would need to pass it as array:
$form->set_modification (array($OtherClass, 'my_function'));
Only note that method should be public ;)