BLUF: This could be a violation of the UCMJ for imposing punishment before administering Article 15 proceedings.
If the Soldier was made to stay after duty hours and clean the COF because of the DUI, and then later was given an Article 15 for the DUI, then yes, his rights were violated. The issue I see is that normally, if a Soldier gets a DUI in the States, then the Army does not give an Article 15 for a DUI off-base because it could be considered double-jeopardy. If the DUI happened on base, then the punishment is at the discretion of the Command.
If overseas, then the punishment is normally left to the Command also. Without more details it would be hard to tell. However, if he did get an article 15 after punishment was imposed, (cleaning the COF), his defense attorney should be made aware. The Article 15 would be invalid. But again, without specifics, it cannot be determined if a violation of the UCMJ occurred or not.