My take on it would be that its a question of good manners!
If you are good enough to invite your mate it would be poor form of you to expect him to pay unless that is what was agreed in advance. By contrast he should have the decency to realise that flying is an expensive hobby and should make the offer.
if the arrangements dont suit both parties they wont last ie you wont have his company and he wont get a jolly.
If you feel that he's taking advantage dont invite him again if he feels ripped off he wont want to come.
Most pilots I take up offer most non pilots dont.
The kitchen sink analagies are poor, a better one harks back to my rugby days . For away matches the driver always got paid at a rate agreed for all by the captain + got free beer.
Sounds like yer mates tight.