Ruthie could be picking up (silent to us) signs from Tango that SHE is boss, especially over the food.
Perhaps your mom could see what happens if she puts Ruthies food down after Tango has eaten (and make certain that Tango leaves the room).
I realize that its also when your mom is fixing food for herself, but Tango could be "saying" to Ruthie-- "mine, get out." So she is. It would be interesting to see what would happen if Tango was told to stay out of the kitchen. Would Ruthie enter? Or remain behind????
hth a bit. Sorry, no definitive answers

good luck,
toodles