Well O.F. you are a gambler and should understand odds.
So if it was a little of both, factor in how much it was each way and do the math. If it was 50/50 then you could credit him 800. If it were 60(your)/40(him) then credit him 640, and so on.
I too have taken bets for guys, way back when (before cell phones)I used to go to the track daily, and used to bet the double and a few exactas for this guy, one day I didn't get in in time to bet the first race, and the guys horse won, and of course he hit the thing. I paid him. I told him what happened, but I still paid him.
He gave the gratuitous "no you don't have to" but he had called to get the results and already had the money spent in his mind. So I was screwed basically. Ethically I didn't really have to pay him, (my ethics anyways), but I did. Had the thing lost I probably would have given him his money back and said I didn't get there in time. But that is the way I am.
BUT, from then on I booked a few of his bets myself, something I had never done before. So it was a trade off. I told him I was going to book them, and he understood. See, gamblers that gamble all the time understand these things. He knew, without me telling him I had to recoup that missed play. If/when he won I still paid, but in the end I was probably further ahead.
But it is up to you. Only you know what kind of nuances you and this guy have together. And with a decision like this, that isn't so cut and dry those things have to weigh into whatever decision you ultimately make.