Yes, this is realistic. If the bullet passes through the body very easily then there will be no backward movement, but if the body offers resistance it will move backwards. In perfectly inelastic collision the bullet will get stuck to the man and maximum momentum transfer occurs.
mass of bullet, m₁ = 20. g = 0.020 kg
bullet's initial velocity, u₁ = 870 m/s .(Here we are assuming that the direction of bullet's velocity is positive, while any object moving opposite to it will be negative)
mass of man, m₂ = 80 kg
initial velocity of mas, u₂ = 0 ( as there is no mention of man's movement, he is assumed to be stationary)
final velocity of bullet & man , v = ?
In inelastic collision momentum is conserved
m₁u₁ + m₂u₂ = (m₁+m₂)v
(0.020*870) + (80.0*0) = (0.020+80.0)v
v = (17.4 + 0)/(80.02)
v = 0.217 m/s
Notice that velocity is positive so the man is moving in the same direction of the bullet. If the man is facing the bullet then he will move backwards