The media/hillary talking points will be along the lines of Trump is such a joke candidate, shes not going to give him any credibility/validity by debating him.
I don't see any way she could avoid participating in the debates. If she does try something along these lines, Trump should respond by insisting the debates go forward with himself, Gary Johnson, and Jill Stein. Johnson and Stein would love the media exposure, and one of them may generate enough support from the Bernie Sanders fans to relegate Hillary to a third place finish.