Radiation oncologist here. It's impossible for me to say much, but his drop in estimated survival rate is probably not strictly related to his tumor being infected. However, he probably has some sort of genetic predisposition to have cancer at such a young age, which can also have an impact on survival. That being said, a few things to keep in mind: survival estimates are notoriously bad. Moreover, oncologists tend to do a poor job of communicating those estimates. 35% chances of being alive in 2 years? in 5 years? alive but living with "active" cancer? Very important points.

A reasonable thing to do would be to ask his oncologist "if there are any other options he could think of". It's simple, but usually it forces us to reconsider the problem from scratch. I must warn you though that in a 19 y.o., we (oncologists) usually think very hard and rarely leave any stone unturned.

From experience, the best a friend can do is simply be there and "act normal" ie don't overdo it. No need to do anything special.

Good luck -- sometimes luck is all it takes