No. She hasn't.
That Black Panther thing was from Marvel Knights, which isn't canon. This whole thing is just laziness from writers who refuse to do anything new and worthwhile with her, so just make her be a trophy for Namor and Reed to fight over. It's time to put that long dead plot to rest.
To reiterate: Sue has never cheated on Reed. I'm tired of writers trying to bring up old Namor plots instead of actually doing something good with the character for once. She doesn't want Namor and it's well overdue for Marvel to elevate her beyond being a supporting character to Reed, being anchored to the FF. It's annoying and I'm tired of it.
I think I might barf.
Whatever writer said that was talking crap. Who said it?