Is guanabana soursop safe to take? What about if you have lung cancer?

Yes. In general natural fruit juices without any concentrates or additives are ok and possibly good; certain fruit extracts like grape fruit may interact with certain metabolic pathways in the liver and affect the efficacy/toxicity of cancer treatments; check with your oncologist during the treatments to make sure he/she is aware of such supplements.
Yes if you like it. Guanabana grows in south asia , south ameica , also called jamaican soursop , rich in vit c b 1 ;2 , has unproven claim of anti- cancer property , with sour taste. Has nutritional value , using along lung cancer medications as a supplement , will not harm . Good luck.