Bernie Sanders endorses Hillary Clinton for President

[Tiberius]

I'd imagine this will split the supporters down the middle, but I doubt many will vote for Trump in retaliation. They are more likely to just not vote for either candidate, or vote third party.

Surely them not voting for either candidate only helps Trump?

It really depends on which state they are in. If they are in a swing state, then not voting will help Trump. If they are in a red or blue state, the effect will likely be less helpful (because Trump will win / lose those states regardless).