Two potential issues jump out at me!
Of course it could be motor control through perhaps a quad dominant tendency where your client can’t recruit proper glut engagement. This would cause the knees to come forward and heels to lift.
The more likely, in my experience, is a ROM issue in the ankle. Try looking into release dynamic stretches that work ankle mobility in the beginning of your sessions, using exercises the encourage the full use of your client’s current ankle ROM, and utilizing some release techniques on the soleus, gastroc and plantar region (foam rolls, trigger point balls, lacrosse/tennis balls, etc.). You might do a little work on the anterior portion of the lower extremity, too!
TRY THIS TEST: have your client use something stable to grab onto (ex. smith machine bar) as they carefully lower with good form into a full depth squat. If they can’t get into a full the end range of a full depth squat (be safe and define what each clients’ safe depth is) and stay there without support there’s a ROM issue (ankle, hips, quads, hamstrings, etc.). In this case, my guess is you will see your client get to a certain depth and either lift the heels or fall backwards (if they were to let go).
Overweight vs Obese – my assumption is that you are talking about someone who is NOT obese.
Hope this helps!