The word 'hard' is a relative term and means different things to different people.
The susceptibility of a particular cornea to the effects of eye rubbing is also not quantifiable at the moment. That is partly why although there is an association between eye rubbing and say, keratoconus, many people do not get the condition despite eye rubbing.
But in general, it is not advisable to rub the eyes 'hard', regardless of whether any surgery has been done or not. Rubbing can sometimes perpetuate a vicious cycle whereby a bit of rubbing causes some itching / discomfort, and then cause the person to rub even more. Besides potentially stressing the cornea, really hard rubbing can also stress the zonular ligaments of the eye.
Do note that we are talking about rubbing with pressure on the eyeball itself here. Rubbing the eyelids over the eyebrow or other bony areas is completely harmless.
