I disagree with the other posters, I wouldn't be cool with that and I don't think it's necessarily "normal" even if it's what some people do. I have a busy life and I NEED to be able to plan things in advance. Ever once in a while a "we'll play it by ear" is ok, but in general I need any partner of mine to be willing to make plans with me and stick to them. After all, we all make plans with our friends and stick to them, shouldn't a partner get the same treatment or better??
Maybe Friday night is too much to ask for if that's often a special, flexible night for him and his wife to be spontaneous, but if so you two should just pick a different day that he CAN plan for in advance.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.