The problem of impact ionization of two Rb atoms, one of which belongs to the Rydberg P series and the other of which is in the ground state, is studied and solved with the use of the methods of stochastic dynamics for description of the path instability of a Rydberg electron in a collision diatomic complex. The actual potential of the atomic framework was taken into account in terms of the single-channel quantum-defect method, whose accuracy was demonstrated on the example of determination of the photoionization cross sections in the quasi-classical approximation. A representation of the effective average lifetime of an external electron in the bound state that takes into account both the diffusion migration within the Coulomb crowding of the potential energy curves for the quasi-molecular complex and the direct quantum autoionization is obtained in analytical form. It is shown that the rate constants of diffusion collisional ionization calculated by us are in good agreement with the experimental data.