OK, leaving a fallen comrade unburied, in a place known to have undead is just ... unwise and rather stupid. If the paladin character had a low Int, I might excuse the initial mistake. But, the (presumably) higher Wis should kick in and make the Paladin stop and think about it.
Out of curiosity, why does the player want to play a Paladin? Is it because of kewl abilities or because he wants to RP out the rather challenging role? If it is for the kewl factor, then you probably have a mis-match for the player/character. The player will probably treat the Paladin's Code as flavorful window dressing that shouldn't really have any affect on the game. When I have a player like that, I give them fair warning that the Paladin's Code is real and will definitely come into play. If they don't pay attention and try to adhere to the Code, I let their actions bring them from Grace.
If the player is truly interested in the RP, I work with them extensively. Admittedly, I run a home-brew game and if the Paladin chooses a God, they need information from me. I am slowly trying to put together player guides to my gods, but it isn't always my highest priority when there is other game work to be done. As an example, in my game, it is generally a no-no to loot a fellow paladin. Your comrade's armor, weapons and other equipment should be returned to his Lord, his Church, his family, or whatever. We are talking about Lawful characters here. Unless you have been given permission by somebody with authority, you do not take that equipment. This is a concept that is difficult for somebody new to my games. The experienced players probably know that I will reward good play in one form or another, so they may be willing to have more patience. You indicate that this is a relatively new player, so maybe you have similar issues.
In any event, I still think that communication is the key. Perhaps the player just assumed that the character would bury the dead. Maybe the player is not accustomed to stating his non-combat actions? I know that might sound weird, but I have had many times where a player doesn't state the character's actions and I will point blank ask them "Are you really not going to do X?" The player may stare at me blankly a few moments before saying "Yeah, he would do that." Then I will say "Well, you need to tell me that. If you don't say it, you didn't do it." I am talking about people that worked out to be great players, they just weren't accustomed to thinking some things through. As well, sometimes people have off nights and are a bit ... spacy.
Obviously, you have a much better idea of the circumstances than I do. I just wanted to point out some possibilities.
Good luck with resolving the issue.
