This is actually rather easy to answer; Follow your Sensei's example. It is your Sensei's dojo and so when there, follow the example set before you.
I suggest that you try not offer to 'teach' how to improve the technique to this individual during training unless they ask for it.
Take what you have said about this person to us and consider it simply: "because he was my teacher in another martial art for about 4 months".
This could mean that this individual while curious about Aikido, has been an instructor in their own right albeit in another martial art and so taking any advice from anyone they cannot see as equal or their senior 'may' default to advice not worth listening to. This may be even worse in your case as you used to train under this person but left his school to focus on Aikido. A choice you made for yourself however has had this possible side effect. You will 'know' the answer to that better than any of us though.
This individual currently has a preconceived 'way' of wanting to train, learn and come to understand what is being taught based on how they have done so in thier life of learning and teaching martial arts. They may simply feel that they know better even if their knowledge comes from another path that they are trying to merge with this one.
Think of it in terms of the path of Aiki. Do not offer any resistance to this individuals 'way' of learning. Do not encourage it with others in your dojo as I am hopeful your Sensei has not done so.
Instead try lead by example. Do not avoid training with them as that is not the best path for yourself if your goal is to help this person. Instead just train with them. Do the technique as you know how without 'teaching it' and see how he goes. Simply encourage him with positive comments when you notice any improvement with his technique. I would avoid any negative commentary regardless of what you see being done incorrectly.
Simply try: "That's pretty good. Try it like this and see if it is easier for you." Then demonstrate what you mean.
However be careful not get caught up with 'judging' his performance in the techniques. It is not important. Keeping your true centre (inner peace) is all that is important and setting the example. Provide an environment that allows him to learn what he needs to his way even if you find yourself leading him 'your way'.
You do Aikido for yourself and when he wants your help, he will ask for it. Perhaps all you need to do is let him know that you are there to help should he ever feel he needs it.
I hope this helps