Personally, I would keep the tri's w/ shoulders. Here is my reasoning. tri's are hit very hard on chest day, they are also hit hard on shoulder day, but not to the same extent as on chest day. So if you do them w/ delts, they are still some what fresh and ready to go.
If he moves them to back day, they are going to be trained 3x a week (1 direct, 2indirect). I don't feel that they will have enough time to recover.