My friend it turns out is basically subletting the place from somebody who he doesn't really know that well, and he doesn't feel comfortable with giving the fish away. Without my being able to convince him otherwise, I'm in a really frustrating situation...
Anyway, I visited the tanks, they were dirty and being unplanted and overfed, they were filthy and in desperate need of a water change. Did a large water change on the pleco tank, which has about a 42 gallon capacity, about a meter long. The other tank is tiny, not even 20 gallons, and clearly unsuitable for the knives. As an interim solution, we decided it best that we try to see if the plecos and the knives will be able to share the space in the larger tank - it will rather overstocked but at the very least the knives will have enough space to maneuver reasonably freely. I am going back there next week and help with the transition and carefully monitor the situation. If my friend is unwilling to give away the fish, I figure the next best thing would be to train him to do water changes and make sure the knives have adequate hiding places, and advise him on some floating plants he can introduce to the larger tank.