Fasting on Friday by eating fish instead of meat used to be mandatory and now is voluntary.
Was eating meat on Friday a mortal sin in the past? Yes.
Is it mortal sin now? No.
Although abstinence from meat on most Fridays is no longer mandatory, some meaningful penitential act is still mandatory. This could certainly include abstinence from meat, and for many Catholics it does. Moreover, during Lent, abstinence from meat is
still required of all but the sick and elderly. I'm surprised that a self-identified Catholic doesn't know this.
Then as now, the mortal sin isn't in breaking an arbitrary juridical ordinance; the sin is in willfully refusing the Church's call to penance. The sin is for pride and disobediance, not for the act of eating.
I don't know how you came to acquire your primitive understanding of Catholicism, but it's time you outgrew it.