There is a lot of potential here.
Who would have the nerve/authority/gall to take on magi? Anyone who also has a personality that encourages support. That is, this guy could be a proper idiot with no idea how powerful magi are, no status or position in the community, but if he can get the entire village to say 'how high?" when he yells 'jump', then he's a problem.
However it's likely more interesting if the person is a person with a position in society. Because the senario has me asking several questions.
First, why can't the visiting covenant solve this problem by theirself?
The easy answer is the number are against them. Yes they could flee, but their covenant would likely be burned to the ground and that can't happen. So why don't they use magic on this guy and change his mind or something?
Well first maybe he's protected my magic resistance, be is the Virtue or a variant such the divine or maybe infernal. Maybe the leader is secretly a hedgie who wants to steal not only the books of the covenant, but the entire covenant. If he can take out the owners, then as the leader of this local inquisition he could take access to all the 'products of the devil' and 'study them to know their evil ways'.
So maybe a problem of numbers and possible magic resistance keep the magi from stopping the guy. But maybe it's more personal. I always enjoy a personal angle because it makes a situation much more difficult that blasting the guy away. Maybe, the leader is illegitmate son of the leader of the covenant and the covenants leader will not allow magi to harm the man or even work magi on him. Maybe the covenant has performed diabolicism in the past. They have recanted their evil ways, but as a group they feel guilt and merit the punishment for crimes not yet discovered.
Now why get involved?
Well my campaign is fill with proper bastards. If someone was tossing words around like 'burn the witch', they wouldn't get invovled. Actually this is a great point because if the crowds are looking for blood this may force the magi to use subtled magics rather than blasting powers, which will turn on them as the crowd out number them.
But the proper bastards I play with might try to steal the library as thier peers rot in jail. While not a 'good guy' adventure it is an adventure and to the taste of my 'bad guy' players.
But in a troupe that is more good guy than bad guy, there are all sorts of reasons to get involved. First, always always consider lineage. Nothing is tighter than a family bond. Perhaps one of the magi shares the same Pater as a character, they are in effect a 'brother' or 'sister'. Perhaps one of the magi is a Pater, or a former apprentice depending on age. Maybe it's an actual blood related family member.
Beyond this maybe they own the troupe vis or a project. Or maybe the troupe realizes that if they save these misfits they will have a covenant in thier debt, which can be exploited later. This is the only way I could likely get my proper bastards to 'the right thing'. The opportunity to exploit them later!