You have to change your life style, diet and this under control of certified dietician. So I see it as a first step. Then, you may consider starting very light running every day, also following advices qualified coach.
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."