I would refer to the following topic about binding a familiar with a soul.
Although there is no final agreement (as with all discussions on this forum 
), there are some common ground:
- it is not possible to bind human as familiar through usual process: depending on the opinion, it is simply impossible, it requires a specific virtue (through some form of initiation) or it requires a breakthrough;
- binding a familiar requires that both side agree and trust each other completely (there is some comments regarding demon familiar, but as usual, there will always be some fringe cases).
There is no agreement if binding human as familiar is impossible because of the soul, tampering with free will or just because human is a human and not a beast (but since with the right virtue you can bind ghost/spirit, this argument is a bit wobbly).
So as usual, my take is: if it is going to lead to an interesting story, then the mage can look for some Mystery Cult with this knowledge. Here are some proposals:
- Provence Tribunal with the concept of Amour Courtois could be a possibility
- any Cult with a strong emphasize on brotherhood/friendship/faithful to the death
- a secret faction of Tytaly who is pushing to the extreme limit the concept of Beloved Rivals - but obviously, the trust aspect required for a Familiar bond must be worked out to fit this twisted concept
- The Order of the Green Cockerel could have a branch which consider that for the creation of the philosophical stone, the alchemist must be made whole by finding his/her soulmate
- integration of a pagan wedding ritual (or maybe St Nerius followers know a Hermetic wedding ceremony)
- a faction of Verditius able to bind a master craftsman in a kind of disciple/master bond, until the disciple is finally able to surpass the master (Jerbiton might have something similar with artist or muse) - the concept of lasting but not permanent familiar bonds is not a novelty since Faerie familiar have fading bonds.
... but obviously, if the purpose of binding human is only to achieve more powerful/broken stuff that is already possible, then of course, it is doomed to fail/corrupt the mage or just plain "NO, BECAUSE I SAID SO" (not very constructive, but some players only understand that).