Suppose the following situation:
Reuvain is a religious Jew who runs a barber shop. One day Shimon, a non-religious Jew, walks into Reuvain's barber shop and sits down for a haircut. Shimon asks for a haircut that would be prohibited by Torah, as it completely shaves the sideburns. Would Reuvain be allowed to cut Shimon's hair this way?
Of course, we're going to assume that shaving the sideburns is against the Torah, as I'm pretty sure there are many who do not agree this is what the Torah meant in Vayikra 19:27 when it said, "do not round the edges of your beard." So really just any haircut that would shave the peyos area, wherever that may be according to your religious interpretation.