The authors actually discuss this possibility in their paper. (Section 4 of [1].) The star is very faint (an M dwarf), so even at its nearest approach it would have been far too faint to be seen with the naked eye. However, M dwarfs do show occasional flares, so it is possible that it would have brightened to be visible to the naked eye for a few minutes to hours during a flare event.
[1]: http://arxiv.org/abs/1502.04655