It's an ethical dilemma as it's not a fair situation to the table as a whole, but you're not cheating either.
I think the important part is notifying this player about his cards being open to you, and doing it in such a way that the other players and dealer knows it aswell. This way, if anything happens against the rules, it's the dealer/floor's responsibility to act and not yours and you fullfilled your ethic 'duty'.
Let's polarize the situation a bit for a better understanding, the player knows he's showing cards and doesn't care. Is this unfair? What if he actively holds his cards in front of your face and waves them around for you to see? Should you look away? Should you step out of every hand he does it in? It's basicly the same as showing his cards in your peripheral vision, he knowingly does it and he himself compromises his hand in your view, it's free information given to you. Players are allowed to lay their cards open during the hand at any time aswell, you're just lucky to be in a position to be able to see them while the rest is not.
What makes it that it's your responsibility to ensure a fair game for everyone else? Isn't it their own responsibility to either keep the seat and play or change tables? This isn't your responsibility, fair play is is ensured by the floor.
Every live pokerplayer is looking for information on their opponents cards. What if you know specific tells on a player when he has AA or KK? You don't share that information with anyone either, obviously. So why should you share or avoid this when villain does it knowingly?
So basicly, unless there is a specific rule on that specific table that forbids you peeking around, you aren't cheating and it's the floors responsibility to act if they seem fit. And you should use the information given for free.
So yes, after notifying everyone, I would look and use it.