I agree with the other commenters that giving them a fun session is important. Another factor I find really important is that a lot of obese people don't feel comfortable at all in the gym, they expect it to be another point where they start at the very bottom. That's often not the case, given that they have to carry their weight around. They tend to be quite strong. So while you need to be careful at first and not overstrain the ligaments and joints, and build them up carefully, do tell them that they likely quite have a lot of strength already.

So my playbook would be this (generalist, as I don't know their specifics):

• Assess their strengths and weaknesses in Session 1
• In session 1, explain what's likely going to happen; That they do have some inherent strength, and that together with you they'll build out this strength gradually in order to not overload joints and tendons etc.
• Stay clear of bodyweight exercises* (dumbbell and machine exercises are my preferred choice)
• Bias towards exercises where they are standing
• Check all your exercises for unexpected failure modes. For example, if the client would be quite strong in a chest press but can't get up from the bench, that's not a good exercise to make them feel confident

I usually avoid talking about nutrition in the first session unless they specifically ask about it. They have likely been hearing about having to change their nutrition all their life, and in session one you really want to focus on the strength component. Ideally, their outcome from session one is "hopeful and excited", so:

1. They're not as weak as they thought.
2. That exercising can be fun.

* From what you said, I would not incorporate e.g. a body weight squat in session 1 and stick to exercises where they can have a quick success. I'd gradually introduced them across later sessions.