In training, as uke, I sometimes attack my opponent as it should, with feet standing like it should etc...
But sometimes I put my feet different, not aikido-wise, sometimes by mistake.
Than I hear my opponent say: "hey, you put your feet the wrong way".
Than I think: "in a real attack, are you going to tell your opponent first to put his feet the right way?".
May your friend, in that setting, change the technique he is using?
Most times, when you train a technique in the dojo, you do it in a Kata manner: Sensei is defining a very specific situation as the Attack of Uke, and then Tori is expected to respond to that situation with a specific technique variation.
From my experience, as you will progress and feel more sensitive to Uke, you may start feeling that even a minor change of Uke stance calls for a change in the technique / variation. Not to mention directions of applied force and\or footing. Once one changes then so should the other.
Same things for attacks, in real life it's more aggressive and with kicks etc... Not of those things happen in Aikido to train on.
Depending on the dojo, "those things" may be part of the things you train on
I dislike seeing Uke ready to jump. Though at times, when demonstrating as Uke to Sensei I act the same way, and do my best to be as soft and sensitive as possible, for very selfish reasons.