How is someone suppose to act when it comes to seeing a friend living with cancer?

Act yourself. Be supportive and let them know you are there for them. Give them a hug and enjoy their friendship. Be there to listen and show them you care. Understand when they are down or not as patient or tired. In short, treat them like a friend.
Compassion and. Support. Your friend can use your compassion and support in many tasks of life, including transportation to and from treatment sessions, taking care of the pet and children and just being there.