I don't know.....Mom, can this be proven?
Yes. There are many witnesses to conversations about the Kerry campaign. And there was an article in the Vacaville reporter about planning to do an ad for the Kerry campaign. I found this website so far, that also establishes the date after the Kerry campaign was in contact with Sheehan:
http://www.usatoday.com/news/politicselections/nation/president/2004-09-29-ad-barrage_x.htm
I also have telephone bills of when I spoke with the Kerry campaign and they were recruiting family members for their campaign.