A family friend had brain cancer(glioblastoma multiforme) at 14. was cured and is now 35 and perfectly healthy. is this rare? is he extremely lucky?

Yes, rare to survive: Your friend had the advantage of age (at 14, quite young, & so the brain still had "plasticity"). Most patients w/ Glioblastoma Multiforme (GBM, or Astrocytoma Grade IV) don't survive more than 1 yr. Less than 1 in 50 (more like 1 in 1000) live past 5 years even with surgery & treatment. Of course, it depends on the size of the tumor & where it's located, but consider ur friend very lucky. Congrat

Surprising: Don't challenge him on this, and don't argue. This is so uncommon that I'd consider the original diagnosis to be in serious doubt, or some other error introduced into the story for some reason. I've been in medicine for a long time, investigated a few remarkable claims and was disappointed, and know how hard it is to get an accurate history. Just tell him you're glad he is better.

Extremely: Overall, it is extremely rare to survive greater than 5 years with a confirmed diagnosis of glioblastoma multiforme. It raises the question of whether the original pathology was correct.