First of all, I have no idea who Lara even is.

Second of all, if my child had a severe food allergy I would think I would be diligent enough to check and see what someone gave them. If it was an event where I couldn't personally be there to check what my child was getting I'd make darned sure that the people over the event knew the situation. And if my child was 10 I guess I would assume he's old enough to know what he can and can't have as well.
But that being said, a food allergy is way different than a 10 year old with braces who receives gum from a neighbor so I don't think the 2 really can be compared. Your son is 10. I think he is probably old enough to learn that there will be disappointments in life. The neighbor was being nice and probably didn't know any better. Just throw the gum out or give it away and quit complaining and defending why the neighbor was wrong on this thread. I think it's time to let it go and move on.
I would love to stay and discuss this some more but I have my own child to tend to. Oh, and she's not quite 3 and I don't let her have gum either. That being said, she receives gum from people on nearly every holiday. I just tell her she can't have it because she is still too little and give her something else if there is something else available (otherwise she just does without). Life goes on and we survive. Your son will too, I promise! Have a wonderful Easter!