Generally speaking circuit training would be best. I’m guessing your client is type II since you said they were overweight. you didn’t mention if they were taking insulin or other medications yet or not. that may be a factor to consider. if they are, I recommend taking their blood sugar pre/post exercise to see how exercise is effecting them.
If you know their A1C levels great, have them get it checked again in 3-4 months to see if it goes down.
They need to burn calories as this will help cure their type II diabetes. that is why I mentioned circuit training. it will do the most good in the least time.
since they have diabetes, they likely have other issues which you may have to consider – kidney issues, joint problems/arthritis etc.
Remember that exercise alone will not probably help their diabetes to the point where they are no longer considered “diabetic”. They have to reduce the calories they are eating too. They must lost some weight if we are to cure this condition.
Yes, I AM using the word “Cure” on purpose because its a word nobody wants to use. For those who dont like this word, I ask , what do you call a person who is off all diabetes medications? I call that a cure. I’ve seen it happen several times so I know its possible.
Here is something I wrote on diabetes that may help