Should I focus on losing fat first before building muscle or the opposite.
Karin’s answer is right on!
My first reaction after reading your question was similar to Karin’s; both! As your client incorporates resistance training and builds muscle, (s)he will start losing fat.
A discussion about diet is not always easy but something to address. I like to point out the need for a calorie deficit and say, “you can burn 300 calories by being on the treadmill for 30 minutes or not drink your typical soda with lunch and dinner. If you do both you’re now at a 600 calorie deficit”. Obviously the example can be adjusted for the client but it show that both are important.
As I’m sure you know, with clients new to exercise or with a lot to lose the big picture is to get them consistently active by creating positive experiences and finding what it is they enjoy.
Good luck!