If they insisted to be served in French then they would have to be served in French, but what little of the situation has been detailed this does not seem to be the case.
I think it's ambiguous. At what point is there a refusal, and what degree of insistence is required? Someone initiates a request for service in French, the response is "no, no, no this will be in English". Has that person refused to conduct the transaction in French?
I suppose the person requesting service could be more forceful about insisting on being served in French, but does someone have to go through a second round of insisting/refusing before service has been denied? And what words count as insisting (id that is required), what words count as refusal? "No, no, no, this will be in English" seems very close to a refusal to me.
I wouldn't be surprised if this has been formally litigated in Canada, it'd be interesting to know.