Your wife's grandfather left the country 5 years prior to the revolution, which created the Red Army, so it must be the Imperial Russian army.
However, shoulder boards are visible on all uniforms, which Red Army didn't use between Dec 1917 and the middle of WWII. This is an indication of Imperial Russian army. If I'm looking at the right person in the photo, he was a private - there are no stripes on his boards, while the man next to him has a single stripe (Gefreiter rank), which is similar or just above of a private E-2 in US (NATO OR-2)
Mike Vayser