Well that depends what they are grieving over and what you mean to them and them to you; the relationship in its current standing
Its most important to let them know you are here for them and always available to talk; i leave my phone by me at all hours of the night and have the volume up, because I make sure all my friends know that no matter what the problem is, no matter what the time of day, all they need to do is call if they need to talk it out :) sometimes I get calls about problems, sometimes they call me at weird hours just to chitchat, even so I always pick up, even if I was sleeping, because you never know what reason they're calling for.. Let them know they can go to you for anything and you'll always there for them, they can call if they need, and you'll provide a shoulder to cry on and a hug if need be :) and if you have good advice for them, giving it is alright too.. make sure they know that its normal to feel what they are feeling, and that they can get through it, and you'll be there to help them as they do and to not give up hope because grieving only goes on for so long before they accept what has happened C: