If you're working with unpaid artists, you're likely to run into this scenario a lot. I do agree that your artist sounds unprofessional and should have been more upfront about their ability to do the work, so it'd be in both of your best interests to break off the current agreement.
However, it's important to think of things realistically from the artist's side. Doing a 10 page one-shot for free is somewhat reasonable if someone has a lot of free time and wants to take on a fun collab. But a 59 chapter comic is an enormous undertaking for even a paid professional team, never mind an unpaid single artist. It's extremely likely that your unpaid artist will eventually need to drop the project in favour of higher priorities.
If you can't afford an artist but really want to do the comic, the best course of action is to build a relationship with a trusted peer and give them an equal stake in the project. This can be really hard with an already-established comic script. But usually an artist will be more involved if they feel like this is their story too, and aren't just a lackey who makes your dreams happen. Not to be rude, but no one is going to love your comic as much as you. This goes for most creators. So unless you can turn "my comic" into "our comic", it's gonna be really tricky finding a devoted volunteer artist.
One more suggestion-- if you're going to work with a new artist, paid or unpaid, it'd be smart to write out a contract. That way you and the artist both know what is expected of each party, and what will happen if either end fails to deliver on the agreement.
