By means of preg_match and the like you can only write rules for specific devices (in your case the GT-P5100). What you want to do, however, is to affect only your device (meaning: not all Galaxy Tabs).
I’d vote for setting a cookie and act upon it’s particular value:
setcookie('no_mobile_theme_please', 1, time()+60*60*24*30, "https://wordpress.stackexchange.com/", $_SERVER['SERVER_NAME'], false, false);
Then, you can check for the cookie:
if (isset($_COOKIE['no_mobile_theme_please']) && (1 == $_COOKIE['no_mobile_theme_please'])) ...
If the plugin does not allow for this (i.e., handling an exception cookie), you either have to adapt the code yourself, or you might want to write the author about your issue.
Or maybe the plugin offers you a hook, so that you can realize your exception that way…?