can someone please explain the end to me

No, why would you think that? She introduced John to the Wheelers because John's doctors said he needed socialization. The Wheelers were the only unconventional people she knew who might be a little bit accepting. That backfired when the Wheelers ended up being conventional after all and John flipped out over it.

We can only speculate on why the husband turned down his hearing aid. The movie ends with Milly (the neighbor) and Mrs. Givings (Kathy Bates' character) both trying to bring order back to the story and their lives. Milly befriends the new homeowners and talks about what a good father Frank is like everything is normal. Mrs. Givings is a real estate agent and thinks in terms of the house. Now that the house has a conventional couple, she's pleased. To reassure herself that the house, and the new family living in it, will be happy and safe, she starts critiquing the Wheelers. Their flaws prove why the tragedy occurred. (She may also be convincing herself that they caused the incident with her son.)

One husband (Shep) walks away in disgust. The other husband turns down his hearing aid. As ugly as the tragedy was, they don't want to listen to it being white-washed because they were both witnesses to it in some way.

I think the point is that the Wheelers were not the only people who felt forced to live a painful fiction. Everyone in that neighborhood, and possibly that time, had to compromise themselves in some way for the social fabric to hold together.