Hmm.. tough one. Like said above, petrol is expensive. But then again I wouldn't offer a friend a ride if I was going to make them pay for it, I'm not a frickin' Uber. So I'd probably be a bit offended too. If he does it every time, I'd be even more offended and would stop taking his offers. But if he was just strapped for cash this once, I'd let it go. How much did you actually give him?