I'm thinking of suggesting we don't offer the flatmate's friend the food we cook
Why do you not want to offer them food?
The guy's friend is not hygienic
You shouldn't be rude to guests in your home. If the friend doesn't have good hygiene, you should discuss it with them instead of being rude.
That's what I say fuck you.
I don't think you would want someone to be rude to you if you were visiting them? Maybe you can have a little more empathy for the friend and consider how you would want to be treated if you were a guest in someone's house?