Sticky Situation

Anyone have advice on how to deal with troublesome neighbors? I don't even know where to begin. We have a neighbor who has kids around my kids ages. One is 10 and one is 5. The 10 year old is a girl and the 5 year old is a boy. The boy just started to play with Jacob. The girl does not like to play with another neighbor girl and Emily is friends with both. Which makes for jealousy. Emily does her best not to offend either of the girls. And tries to include both girls in play. Three is a crowd as they say. We have already had issues with the parents with the above mentioned kids.
Now this afternoon I get a call from the mother telling me Jacob was not being very nice to her son at the bus stop. She says "it's apparent Jacob no longer wants to be friends with my son" and that"He was good enough to play with a month a go" It all stems from yesterday,her son asked Jacob to play,Jacob was already playing with other kids and said he didn't want to play with her son. Jacob didn't say it in a nice way and I told Jacob not to speak to friends that way. All I can say is kids will be kids and you correct them when they get out of line. But some people think their kids should get their way. As seems to be the case here. I understand kids sometimes do and say things they don't mean. But I don't think they do.