Why is entropy change in an irreversible adiabatic process positive?
The friction they are talking about is viscous friction of the gas itself.  This comes into play when the gas is deformed rapidly, and is roughly proportional to the square of the rate of gas deformation.
Regarding entropy, the change is entropy is equal to the integral of dq/T only for a reversible process.  In an irreversible process, in addition to this exchange of entropy with the surroundings, there is also entropy generated within the system itself.