I am so sorry to hear of the death of one of your friends children @Clawde 

There is no easy way to deal with the death of a loved one.

I am sure that your friend understands that there really is nothing much anyone can say to ease the pain of loss.  It takes time to work through the stages of grief in their own way.  

Perhaps you could just say how terribly sorry you are to learn of the death of their child, and give them a hug.

I think that means as much as anything else at such a time.  And then maybe let them know that if they need anything, you are there.

I agree with you that asking RUOK in this circumstance is perhaps not appropriate. Because you know they arent. How could they be?  But in months to come, it would not hurt to ask that question as they come to terms  with their loss.

Not sure if any of this helps.  I know you are a sensitive and kindly soul and want to say and do the right things for your friend.  But its different for everyone when it comes to loss and grief.  I'm sure you will do the right thing.  As I said, nothing beats a genuine and heartfelt hug, and a shoulder to cry on.

Emelia 💞