There is zero evidence suggesting the Fredo knew about the hit. Forget about the boathouse when Ola called him he was clearly upset with him, “You guys lied to me”. Being resentful doesn’t equate trying to kill your brother. Like I mentioned earlier I understand the need to try and find some justification for Mike murdering Fredo. The truth is Roth used his stupidity for his own gain but it’s pretty clear to me that Fredo had no idea Mike and Kay were targets to be killed