Catalyzes the formation of a C-F bond by combining S-adenosyl-L-methionine (SAM) and fluoride to generate 5'-fluoro-5'-deoxyadenosine (5'-FDA) and L-methionine. Can also catalyze the hydrolysis of SAM into adenosine and L-methionine, and halogenation of SAM with chloride to generate 5'-chloro-5'-deoxyadenosine (5'-CIDA) and L-methionine. In vitro, in the presence of L-methionine, can catalyze the two-step conversion of the non-native substrate 5'-CIDA into 5'-FDA, via the intermediate SAM