Once a week we visit the mother-in-law. We assemble new furniture. Play games with her. Have dinner together. Just chat.

She can be very grumbly. This really bugs the husband. I can listen to her complaints and not respond. The husband needs to pushback-- to give her feedback. This frequently escalates to verbal arguments. 

I'm not going to fault the husband for this. He has probably had to push back against his mother since childhood least she steam role him. His emotions don't quit understand that she is now powerless over him, unless he gives her power.