You mean a pre workout supplement?
If you read the labels you can see that many of these supplements recommend against taking them if you have high blood pressure or diabetes. Both of these are common in the obese population. In fact there is a long list of health problems in which you are recommended against using these substances. You are also warned usually against ingesting any other source of caffeine while taking them, as well as not to take them if you have high blood pressure. For the health of your clients and the sake of your professional liability you need to be aware of all warnings, and all health risks. Even aspirin use is not recommended.
Remember that caffeine and DMAA rev you up. Maybe good for workout, but straining on the heart. If one is carrying significant extra weight that is also a strain.
There are also questions of professional scope of practice with any sort of supplement recommendations.
The use of supplements also promotes the mind set of ‘I am going to do this really fast’ rather than this is a lifelong journey. That mindset tends to make it easier to get crazy and loose energy and burn out, or get an injury, rather than taking the slow and steady long haul approach.
There are some times that certain kinds of supplements may be useful. And people who are strong and healthy and who have done their due diligence to figure out what they are taking, why they are taking it, and what the possible effects are may in some cases benefit from them. But I do not think there is a great benefit to be had for someone who is obese.