Fabricator-General is the head of the mechanicum.
It seems Fabricator Locum is the equivalent of a planetary governor.
Fabricator Locum is a position beneath Fabricator-General, kind of a Vice President if you will.
Apparently Forge worlds are under the ruler ship leadership of a high ranking Magos. I will keep looking into this.
Found this:
"En los Mundos Forja del Adeptus Mechanicus, el Gobernador Imperial es solo imperial de nombre. Los Tecnosacerdotes eligen entre sus Magi de mayor rango quién será ascendido a Fabricador General, rango equivalente en el Mechanicus al de Gobernador Planetario. La lealtad del Fabricador General siempre es primero para el Culto Mechanicus y luego para el Imperio, y es notablemente más independiente del Adeptus Terra que otros Gobernadores Planetarios, pues su única obligación para el Imperio es que su mundo entregue los diezmos necesarios en material de guerra y otros artículos manufacturados a su debido tiempo. "
For those not versed in spanish (my native language) it says:
"On the forge worlds of the adeptus mechanicus, the Imperial governor is only Imperial in title. The techpriests choose amongst their ranking magi who will be promoted to the rank of Fabricator-General, the mechanicus equivalent rank to that of planetary governor. The Fabricator-General's loyalty is always to the cult mechanicus first and the Imperium second, and he is notably more independent of the adeptus terra than other planetary governors, his only obligation to the Imperium being that his world delivers the required tithes in war materiel and other manufactured items in due time."
To further clarification, let's see the concept of Fabricator-General
"The Fabricator-General of Mars is the secular political leader of the Adeptus Mechanicus and the Magos Mechanicus or High Priest of the Cult Mechanicus. As the leader of one of the most powerful organisations within the hierarchy of the Imperium of Man, the Fabricator-General maintains a permanent seat on the Imperium's ruling Senatorum Imperialis as one of its reigning High Lords of Terra. "Fabricator-General" is also the title given to the senior Magos who holds sway over an individual Forge World of the Mechanicus by dint of their superior knowledge and insight into their chosen field."
As you see Fabricator-General is the name of the title that any Adeptus Mechanicus leading a forge world gets. It's the effective equivalent of the Imperial planetary governor. However the Fabricator-General of Mars is considered THE Fabricator-General amongst his peers, and the Leader of the Mechanical Cult, so he's the ruler of the Mars Forge World and also the leader of the whole Adeptus Mechanicus.
So your friend wasn't wrong after all it seems.