I've learned over the years that once you discover you are talking with a plastic Top 6 type fan, it's simply not worth talking about football with them.
Talk about the weather, or whatever's in the news, and you might possibly have a conversation worth having.
Someone like your ManUtd freak friend simply would not be able to comprehend why blokes like us follow a team like AFCB for decades, through all the dross, the ups and the downs, the years of mediocrity.
They just wouldn't get it. No point in trying.