Two electrically neutral materials are rubbed together. One acquires a net positive charge. What happens to the other?
Well, electric charge is conserved, so what do you think happened?
We started with neutral materials. The material that gained a positive charge, must have lost a few electrons to the other material, which necessarily gained a net negative charge.
Because the electric force is monumentally large in magnitude, static charges can be developed after the transfer of only a few electrons. We have all noticed the effect of walking on a woolly carpet, and then the discharge when we touch a metal door knob.