If that person was never Catholic, I agree. If that person professed the true faith then committed the sin of heresy, that person is a Catholic with the sin of heresy on his soul. Should he decide to repent, he can go to confession and be absolved, something only members can do. The nature of the sin makes this unlikely.
One must accept that heresy is a sin, the worst of all the sins, but that is what heresy is, a sin, one that Catholics can always be absolved of in confession.
OK, regardless of whether you hold any particular heretic to be Catholic or not, Pope Pius XII teaches that membership in the Church is limited
only those who:
1) Are baptized
And2) Profess the true Faith
And3) Have not been so unfortunate as to separate themselves from the unity of the Body
Or
4) Have not been excluded by legitimate authority for grave faults committed
So, if any one of the four things is lacking a person is not a member of the Church.
Do you agree with this?