Someone in US gave this plant to my friend who lives in Vietnam, to cure his cancer. It was grown in the US. What is this plant?
It looks like kava, Piper methysticum, to me, or more likely Betel (Piper betle), due to the part where the leaf meets the leaf-stem. I could be wrong. I searched for cancer-curing plants, and it popped up. It looks like it. Whether it may help with cancer, I can't say, but there's probably a reason for the association, whether or not it's an all-out cure.
A plant an American sends isn't guaranteed to be an American plant, especially in the realm of alternative medicine.