Here's what I tell my classes: there are house rules and EVERYONE has to be consistent. You need to know what the rules of the house are in order to tell the dog the rules.

Problem here is your future father in law isn't going to change his way of thinking. So no consistency. Also, you are in HIS house, so basically its HIS rules.

Just be happy your future father in law has taken to your pup so beautifully. He's probably thinking he's treating the pups to a great treat. Perhaps a little POLITE information on toxicity and such will change his mind. Perhaps a trip to the pet supply store for him to pick out doggy treats is in order.

The way I look at it? Don't make a big issue of this. Its a little food for a temporary living situation. As long as he's not feeding anything that is toxic to dogs, then let it go. Begging is more about the attention they get from the dinner table than the food. Supplement the human food with good doggy food (less of it though so they don't gain weight).

Is this the best solution? Not for the dog, but it is for the interest of keeping your fiance's parents happy. If someone moved into our house and started dictating how my husband or I should behave in our own house, that guest would be very unwelcome very quickly.