I always had the impression that Fredo just provided Roth with some extra information in order for Roth's supposed deal with Michael to succeed and for himself to make a good figure, not knowing about Roth's plans against Michael. Maybe he even wanted a bit of a little advantage for himself out of the deal, feeling ignored by the family though being the older brother. So I think he was more careless and naive (which Roth used to his advantage) than having actual bad intentions against Michael. At least that's what Fredo says about it to Michael:
Fredo: They've got Pentangeli. That's all I can tell you. I didn't know it was gonna be a hit, Mike. I swear to God, I didn't know
it was gonna be a hit. Johnny Ola bumped into me at Beverly Hills, and
he said that he wanted to talk. He said that you and - and Roth were
in on a - a big deal together and that there was something in it for
me if I could help 'em out. He said that - He said that you were bein'
tough on the negotiations, but if they could get a little help and
close the deal fast, it'd be good for the family.
Michael: You believed that story? You believed that?
Fredo: He said there was somethin' in it for me. On my own...
While we don't neccessarily need to believe him (as he could just be pleading for innocence before Michael), this seems pretty much in line with how his character is depicted throughout the series. He always seemed to me as not the brightest of the brothers and a bit naive. It doesn't seem too unlikely that he would be a bit too careless with information when he thought he could make something "on his own", without deliberately endangering the family.
So I for myself believe his pleading. I don't think he ever intended Michael to be hurt or the family as a whole, even if he maybe wanted to make a little advantage behind his back. This just wouldn't fit to his character as depicted, I think. But this is all just my own impression without much substantial proof and without providing much information of what exactly he told Roth then (and I admit I haven't seen it for quite some time).