First of all, it need not have been a surprise. If Fredo was behind the assassination attempt (even if he didn't think it was an assassination attempt), then the hitmen would likely have trusted him. And while Fredo is an intellectual and emotional midget, he's not so stupid as to not see that those two hitmen, if captured, could very well finger him.

So my version of events is that the two hitmen manage their escape, heading towards the ditch/sewer. They're in a very big hurry, because if they're not well away from the estate pronto they're caught. Fredo meets them there, and because he's a co-conspirator, they don't think anything of it. Being in a hurry, probably even panicking because they've just shot (maybe killed, they have no idea at that point) one of the most powerful crime bosses in the country, even someone like Fredo could take them by surprise. "Hey, fellas, quick this way!" and then blam blam!

And while Fredo certainly was a pretty weak fellow in brains and courage, he had had enough nerve to try to go out on his own by cutting a deal with Ola and Roth. Heck, he had even managed to keep secret his visit to Cuba with Ola.

Would it have taken nerve to kill the assassins. Yes, it would have taken nerve, but even someone like Fredo, confronted with the choice between killing two men and having it revealed that he had conspired with Roth and Ola against Michael, might find a brutal execution a fairly easy thing to do.

And, of course, it got easier after that. Michael left almost immediately after the attempted assassination, so he really wasn't faced with having to spend any time with Michael until a lot later. Even in Cuba, when Fredo arrives with the money, he almost spills the beans to Michael.

So, in a way, Fredo's weak will gave him enough strength to save his own skin.