I'm not convinced you need to pay someone to help you lose a stone of weight or so; you may want to and it may be helpful, but it's by no means required that you have a certified dietician or have a qualified coach to try and eat healthily and do some exercise.
The suggestion that Aikido will not help you lose weight makes a lot of presumptions on the person in question and the dojo they are planning on training at.
The question isn't "should I get back to training as a weight loss and fitness programme" it was "should I get back to training" with the additional question of "will being out of shape be a problem?". My answers are "Yes, if you want to or think you want to. You are free to change your mind at any time." and "Not necessarily; it depends on how you approach your training on the mat and possibly off of it as well if you feel like it's necessary or you wish to."
You probably didn't read carefully TO message:
Problem is, across this period of time, I also gain one stone of weight, huge pot belly, turkey neck and lost my fitness. I basically can't even do a couple of break-falls without panting and the thought of doing a full kote-gaeshi ukemi or a hip throw ukemi is nightmare to me now
It means he is overweighed and no fitness at all. Attempt to practice, particularly intensive practice in such health condition is simply dangerous. Also having idea that aikido practice will help him out is another dangerous illusion.
Human body is much more complicated that car, and for serious problems with your car you simply go to the car expert. Then you have a warranty of a job well done. Then, you can drive safely. Now human body needs even more care that is reason for my advice. The professionals have a well-established plans for such cases. Also, regaining a good health it is a process that takes a time, sometimes years. A follow up with professional is a big advantage.
Aikido has different goals, and shouldn't be misused.