Issues w/ my husbands mom babysitting our son. Help?

I already told my husbands mom that she could keep our son overnight 1 night this weekend. My husband is mad at her this weekend so he wants to tell her that our son was bad & misbehaved and for his punishment he cant stay the night with her. But truthfully its because he is mad at her. I am not mad at her so should I just let him tell her this lie. Or should I have him tell her the truth? Or just go along with him and say that our son was bad.