I think it all boils down to Marcus being one of the early members of the anti-Lisa group in the house, so Freddie will stand by his man even when he knows Marcus has acted like an idiot.
I don't think he tried to influence Bea's opinion as such, but that is what happened in the end. She realised Freddie wasn't about to start slagging off his mate, so she very quickly altered her opinion for fear of it compromising her game plan of being best buddies with the known favourite.
That's my take on it anyway.