Edit: based on seeing it in actual size, I would say it would be hard to see it with the naked eye, that said as pointed out in other comments, I suppose not impossible if a human eye can detect a single photon.
The article did mention that the photo was long exposure, so it may not be possible to see it easily with the naked eye. A single photon emitted occassionally would be pretty hard to see, even though the human eye can detect a single photon.
Edit: based on seeing it in actual size, I would say it would be hard to see it with the naked eye, that said as pointed out in other comments, I suppose not impossible if a human eye can detect a single photon.