There are several issues that are unanswered.

To do a proper aerial mend do have to read the water and correct for the unequal flow. This is easier when you are wading and fixed in the water. But you were in a MOVING boat and the same mends that work when you are stationary, for example, and upstream reach from the middle of stream when casting to the bank, may NOT work from a moving boat.

1. So the first issue is whether your partner was performing the correct aerial mend. You said he was in back and doing a reach cast. If he was casting to the bank, where the flow is slower than the location of the boat, he would need to read the flow of the boat, the flow of the water between the boat and the target, and the flow of the water at the target. Moreover, the guide is rowing upstream AGAINST the flow. So your partner would have to factor that in.

2. I think this was a learning opportunity. First ask your partner whether his mends were working. If they were not, that is your answer. He could not read the various flows so the guide believed seeing the line on the water would allow him to see how he should mend.

3. If the reach cast was reducing drag, then ask him whether he was mending the rod TOWARD the guide. In other words, was he interfering or limiting how the guide could row?