Lets pretend that Matt and Foggy met in undergrad. Matt didn't have a lot of friends growing up. He had less after his blindness. At some point he made up friends, even into his late teens. Then they go to college. Matt 'meets' Foggy, he assumes he had a room alone because of his disability. Sure it was awkward at first, which made Matt believe Foggy was real. Then they get along like a house on fire.

They get along too well.

Matt's other 'friends' disappear with Foggy. So he assumes that, naturally, Foggy is a figment of his imagination. Sure things 'align' but it wouldn't be the first time that Matt's mind has made it seem like his 'friends' and real life aren't mutual.

The he figures out Foggy is real and he's stunned.