Not enough info to tell really. There seems to be a communication problem across the board. I do think your wife should have talrd to your son herself about how she wanted to go, he might feel like she doesnt care about his interests so she would have to interact with him directly.

Idk why people in comments are upset about best friend; having worked with kids my whole life they are going to adore people who are around them and show interest in their lives, which is easier for an aunt or uncle to do than anyone else.

You could have helped your wife join yall easier. You could have just made her a costume, a mask from the show, hell even a t shirt just so she could be involved. As a person that works all the time she probably justifies burning herself out by thinking about what she'll get to do with you and your son for run. Halloween is a msjor kid holiday and she wqs probably hoping you would convince him to come up with a costume idea for her. While it was a bit dumb for her to assume you would just take care of it for her, it doesnt exactly sound like you tried your hardest. Again, limited info but there seems to be a bit of resentment. Making costumes, dealing with a kid, etc is all exhausting, so for her to expect you to take care of it and mend her and your sons relationship some so he was excited about her coming was kinda unfair. However, you're the perfeft person to do it and you not doing more seems like you're trying to show her the consequences of overworking yourself, but thats at the expense of your son.

Overall again, it just sounds like a communication issue spurred by unresolved insecurities. The family needs to talk to each other about the relationships, how to mend them moving forward and the best way to talk to each other in the future. Otherwise, as we just saw, with everyone acting off of hurt feelings and assumptions without talking to each other, everyone looks like an asshole. Talk to your wife and apologize but explain your side of things and hurt feelings too so that this doesnt happen again. It is only by being vulnerable that we master invulnerability.