I checked ancestry.com's pension index. It show John Freitag's wife Louisa filed for a pension on 27 Mar 1866. Her application was #124102 and her certificate was #75059. His unit of service is given as Co H 183 OH Inf (not 175 OH Inf).

I then checked the Civil War Data Systems database and found:
John Freitag enlisted 13 Oct 1864 as Pvt in Co H 183 OH Inf. died of disease 5 Apr 1865 on Hospital Steamer R.C. Wood. Listed as POW (dat and place not stated) (Paroled).
other info: buried Jefferson Barracks Cemetery, St. Louis MO...Gravesite 45-105.

You probably have all this, but the unit of service is different than what you state.

Perhaps the widow had him reinterred (brought home a buried there?).