Possibility is ADHD. He is also in an open triad with his wife & husband. He IS scatterbrained, for sure. For the better part of two years I had to make it a point to email him the night before our evening together just to make sure it was still happening so I wasn't left wondering where he was & why he hadn't called to let me know plans had changed.
Part of it was that he really didn't remember who he had plans with & part of it was that he had gone off his meds & no one knew about it so his memory was a LOT worse than it should have been.
After a lot of my telling him that it would be nice to have at LEAST 24 hours notice when he knows the plans have been changed, so I can make plans with someone else if I like, he has gone the extra step and tries to give me 48 hours notice!
It used to drive me totally bonkers that I couldn't get any undivided attention from him until I realized that I was getting it, just not the way I was accustomed to. He doesn't fulfill my primary love language, touch, but he does fall into a couple of the others. Once I figured that out I felt a lot better.
Dating someone with ADHD does take a lot of planning, repetition & frustration but it can be SO worth the struggle if he's the right one for you! Things are slowly changing for Possibility, for the better.
He's also a HUGE multi-tasker! He can play a video game, watch/listen to a movie & carry on a conversation & never miss a beat with any of it!
I would suggest sitting down with both of them & getting as much information about how HE is, what can be attributed to the ADHD, what's just plain forgetfulness, etc. so you can make a more informed decision as to whether this relationship is the right one for you.
As for the shopping....If you weren't in his direct line of sight he may not have been able to remember you were along, she should have hinted, strongly, that you might like to give your opinion on it as well.
I have found that repetition will generally get me the results I am looking for, if not when I want them, lol. Like I said, nearly two years of a weekly reminder of when our night was supposed to be before he was able to remember it on his own consistently. Even now if I know he's had a really bad week I'll nudge him via email or chat just as a reminder.
I know there are others on here who have experience with this. Hopefully they will chime in soon. Feel free to pm me if you need to vent or something, lol.
There are as many ways to do polyamory as there are people practicing it!