Yes a customer flying on an airplane can purchase the seat next to them if they are heavier in weight and one seat won't fit them comfortably.
But if you only need one seat for yourself then you should not purchase the additional seat so another passenger can have that seat available to them.
I have seen some overweight people sitting in 2 seats and they had to purchase the seats for an additional airline fare which was the same price as the first airplane seat.
Just tell the airline at the ticket counter that you need an additional seat and they will allow you to purchase the additional seat.