Welcome NT.
I'm sorry to hear that this young person is facing such a concerning situation - it must be very hard for them and the family. It's wonderful that you want to reach out to this person and sometimes just knowing that there is someone "there" for them is all that is needed at this time.
My husband has always been very good at "we'll cross that bridge when we get to it" while I tend to be the one that rushes forward following the trail of "what ifs". I have found that his method often saves a lot of grief and worry. This family member will come to what is happening to them on their own terms in their own time; which is often the hardest part for those of us who want to "help". Right now they are just processing the information they have been given and must also wait to find out the direction the direction this diagnosis will lead them. That is a lot to deal with.
I think that just letting them know that if they want to talk about it you're willing and able to listen. They will be comforted to know that they have someone in their "corner" but will also appreciate having someone who is not peppering them with questions or pushing them for a response.
All the best -Razz