I think Boss Fan hit the nail on the head about 'three' kids..... another mom and I discussed this recently. My kid is the 'only' in the situation and she has two children; one who is considerably younger than mine, one the same age. They all play together *fairly* well, but she and I both noticed that the trio requires a parent nearby to help the kid who is the odd one out, however that might occur. (Their ages are 7,7 and 3.5 or so.)
Those 'trio' playtimes need a lot more adult help because, basically, the youngest one often becomes involved in the petty disputes between the older ones. What I see happening is fairly age-appropriate and I do see my son trying out new, socially-inappropriate ways of solving problems. Why? Because he's got new situations and new opportunities which he hasn't had before. All three of the kids need help from time to time, but when the two older ones play alone, things go more easily.
I don't know if it helps you, but it really helps me in my relationship with the neighbor mom -- I talk to my son about his behavior there and then, in the moment. "I understand that XYZ happened, and you can't ask your little friend to do ABC with you to get back at your older friend." If it was ever something *terrible*, of course the playtime would end, immediately. But any time we can provide a chance to come up with new ideas for problem-solving, that's our go-to, talking with the kids (since there is a 3 year old involved, we have to work with this in mind) and a strong warning that if the behavior continues, the playtime will be done, immediately.
For what it's worth, I do see this sort of behavior a LOT amongst the kids at his school. The girls seem pretty overt about the drama of the threesomes, where girls hold 'power' over each other and the strongest is nearly always allowed to 'choose' which friend gets to sit closer, etc. I do think part of it is age, part of it is opportunity. So, you might take a break on the playdates for a while, or just agree with the mom "you know, it seems like all of the kids are testing boundaries... would you like to just do outside playtimes and have us both be present?"
I think parents can be blind to their kids' behavior sometimes; if they don't see it, if they don't usually have that dynamic at their home regularly.
Personally, if it was someone who *kept* telling me my kid was the problem, I likely wouldn't want to send my child, period. Not because he wears a halo, but because I wouldn't want him around another adult who was already anticipating that he was going to be trouble for them.