It's really sad that people are not only insensitive, but intolerant, of the right to assemble and have a peaceful gathering to commemorate the loss of a fallen cyclist.

I don't think there is anything you can say to the mother of the young man that would change her mind. What you can do is set an example by working to educate the public about cycling safety and being responsible drivers. You can accomplish this through an organized ride in the name of the victim, press releases, sponsorship, etc.