  We consider single ionization of helium by fast highly charged ions. The
ionization is treated as a three-body problem which involves the 'active'
target electron, the target core (consisting of the target nucleus and the
'passive' target electron) and the projectile-ion. We show that, unless the
transverse momentum transfer in the collision is large, the deviation of the
interaction between the projectile and the target core from the purely coulomb
law has a weak effect on the ionization. In particular, the account of this
deviation does not enable one to get an agreement with experimental data on the
fully differential cross section for the electron emission into the so called
perpendicular plane.
