As someone who started in the Bernie camp and now finds himself in the Clinton camp, here's my views.
Firstly, I still sort of doubt this will happen. But if it does, I'm gonna try my best not to worry. The fear is that a debate hosted by Fox News and starring Donald Trump will be an opportunity for them to attack Clinton with Sanders help while she is unable to defend herself. If her campaign takes that threat seriously, I think she'd just join the debate so she can defend herself. But I'm going to give Bernie the benefit of the doubt here. I know he doesn't want Clinton to be president, but I know he really really really doesn't want Trump to be President. I think he knows the potential for this to help the Republicans in November and I'm gonna give him some credit and assume he's gonna try and make this debate about him v. Trump, and not him or Trump v. Clinton.
That's my two cents.