Replace that line with the following:
if(!is_object($val) && !is_array($val)){
$this->p->$c = trim(stripslashes($val));
} else {
$this->p->$c = $val;
}
This is just a temporary solution so the error will go away. All this does is check the variable to see if it’s an object or an array. If it is the variable just gets saved to the correct place. This doesn’t fix the root of the problem which is the fact that the user object is attempted to be saved as a string. This may cause other problems.
It is HIGHLY recommended you get in touch with the plugin developer.