"In the week which followed we saw them frequently for they proved extremely useful to us" 
(a) Who is the speaker referring to as them?
(b) How did they prove useful to the speaker?
(c) Give the antonym of 'frequently'

(a) The speaker is referring to the two young boys Nicola and Jacopo. 
(b) They were extremely useful to the speaker and his companion for getting a pack of American cigarettes, seats for the Opera or the name of a good restaurant.
(c) An antonym of 'frequently' is 'rarely'. 
Hope this helps! :)