Cost splitting for private flight?

Hi all:

I have a question about cost splitting for a flight by me, with only a
private certificate (in a rented plane).

It's fairly well known at work that I do have my certificate and I'll
go up on sightseeing flights with my friends if they're interested
(always looking for an excuse to go to a new airport).

On Friday, someone was mentioning that they were going to need to rent
a car one way to go down to Norfolk to pick up their car. Hearing
this, I said 'Well, I can just fly you down there myself.. I've been
wanting to check Norfolk off my 'list' of airports to go to'

And so I flew him down and then flew back by myself (in the
ever-present DC area summer haze). Total time was about 3 hours,
about 1.5 each way.

He has volunteered to give me some money to defray costs. Is he
allowed to pay up to half of the whole flight, half just for the time
he was in the airplane, or none?

I think half just for the time he was in the airplane, but I'd just
like to see how my opinions jibe with that of the peanut gallery here.

Thanks,

aw