I have lost three members of my family this year, so death has been around me. I don't think that anyone can prepare someone else or help someone else cope with it. I think the only thing you can do is help them to know that you are there for them. Not 'I understand how you feel' because unless it was your dad,cousin,uncle (as in my case)...or your aunt with the same type of relationship, than you don't. I am not saying abandon your friend though.

Perhaps is you allow him to talk it out. Don't ask him a lot of pokey questions, but if he has to vent let him vent. Or, if he needs to be distracted so he can deal with whatever issues little by little, go out with him (is you visit him soon) or talk to him about things he in interested. You can tell him that you that his aunt IS 'insert your opinion here' and remind him that she always will be that way. (Unless of course his feeling for her change). Beyond that I can't say much more, cuz it starts getting into 'faith and religion' type stuff, and my views on death probably aren't the same as yours or his.

be there for your friend (like it seems like you are anyway)!