Nobody volunteered to push ads for Google
"ultimately, if somebody is paying you, you can't publicly slag them off and expect the relationship to continue" - well, there's the rub. Was she paid? I thought it was a volunteer job.
I'd be pretty annoyed if I volunteered my time for free only to discover that my work was basically providing cheap tax-dodge advertising for a megacorporation, even if it was an otherwise ethical megacorporation. And Google is a very long way from ethical. I'd imagine she's quite rightly pretty livid.
If Google want to employ someone of her calibre, then they should convince her to sign an employment contract and pay her a salary. Tax-dodging your way around employment law in the name of some horrible half-bred mutation of charity and marketing is not the right way to do it.