Do you think for one minute that he would have gone to 100 ft if the DM had told him not to and kept the whole group at 60 ft? I strongly doubt it.
I have to think its a possibility. It appears that he broke away from the rest of the group at 100', so why wouldn't he have done the same at 60' if an opportunity presented itself.
Do you think the only reason he went below 100' is because the DM didnt tell him not to?