This particular use of CRISPR-Cas9 should be pretty low risk. Tumors are often surrounded by lymphocytes, cells which ordinarily have the power to kill cancer cells -- but many cancers give off signals that cause the lymphocytes to ignore tumors cells and let them grow. It's like the mafia buying off the local cops. Modifying tumor-reactive lymphocytes ex vivo might wake them up and help them do their job and go after the tumor again. In a patient with terminal cancer, this treatment should be pretty low risk and could give him a bit longer to live.