How can I resurrect a player who is dead behind a boss?
You have fallen victim to the not enough pixels demon, preventer of resurrections and smudger of details. Your best chance would come when he's leaning a little to the right, but even that's pretty fine.
If you can't get a good click on that gravestone, no, you cannot resurrect your friend.
This is another example in an appalling list of missing keyboard short cuts.