Bf wants to know what beer ingredient fights cancer?

Likely none: Alcohol is a factor in the development of carcinomas of the mouth, esophagus, liver, breast etc. There is no evidence that drinking beer prevents cancer.

It doesn't: If this has anything to do with reality, it's probably a cell-culture study from an obscure journal. There are hundreds of different molecules in everything we eat, and one or more of them may make some cells grow faster or slower at concentrations that are unrealistic. Tell him to enjoy his beer in moderation.