Because if your father is able to settle the bill. That shows that your father, a person of very good character, is not dilly-dallying, not a person who gives you trouble. Your father's handling of this relationship will have an impact on your status in your mother-in-law's house, and their family's opinion of you.
The issue is really that your father must give, which is showing how your father handles himself. Is it your mother-in-law's problem to accept it or not? Because that's your mother-in-law's way of dealing with people. After all, your mother-in-law's family. Open restaurant is also to make money to support the family. Not let people eat and drink for free. Being a man of the world should be separated.
Your father to your mother-in-law does not receive that shows your father and your mother-in-law's behavior are more upright, more in line with the actual more humane, if your father does not give your mother-in-law and want. If your father does not give your mother-in-law and want it, that means your father and your mother-in-law's people have their own merits.
Personally, I think it's better to give. After all, your father may have more than one friend. If the food is good. You'll be able to go and eat often. If you don't pay the first time, you certainly won't pay the second or third time. This is not good. It will also affect the way your in-laws' family views you.
If the first time your father brought a friend and charged for the food at your mother-in-law's house, then your father would not patronize your mother-in-law's house, and there would be a reason for him to do so. Then in the future, your dad will not go to your mother-in-law's side to eat, your mother-in-law can not say anything. It's better for people to do well for themselves. Personal opinion, thank you!