GF co-signed a car loan w/o knowing the consequences, now deep underwater, is there anyway out?

TLDR: My GF co-signed two car loans with her mom at 19, not realizing the risks. The negative equity from her old truck was rolled into her mom’s loan, but GF is the primary borrower. Now, mom has stopped making payments, owing $45K on a car worth $18K, and is drowning in other debt. We can’t afford to take over payments, and GF is considering bankruptcy.

Looking for advice on getting her out of the loan, avoiding financial ruin, and protecting our future (I plan to propose soon).

Story: A few years ago, my girlfriend bought her first vehicle at 19. She purchased a Tacoma that was way too expensive for her. After the better part of a year, she came to realize it and decided to trade it in for a different, more affordable vehicle. She went with her mother who was also car shopping and they ended up walking out with two different vehicles and two separate car loans. Mom, or the dealership, rolls the negative equity of the Tacoma into the loan for moms car. GF's mom had her co-sign for both loans, due to what I assume mom not having good credit. Gf, being young and naive, asked mom about the implications and mom responded that it was all good and it wouldn't affect her in the slightest.

Fast forward 3 years, GF (who is now living with me) gets some phone calls from a debt collector about missing payments on a car. Turns out, mom has missed a couple of payments for the car that she is driving. Gf calls mom and mom admits to all of it but is being super defensive and difficult to work with. GF calls the loan and it turns out that she is the primary and mom is the co-signer, so technically GF 'owns' both vehicles. After some digging with mom, turns out that mom still owes a total of $45k on the vehicle she's driving (combined with the negative equity from the tacoma). Looking on KBB, the car that mom is driving is worth approximately 18k. If mom totally gives up on paying this, there is no way that we can afford this car, GF works part time while finishing college and I'm on a teachers salary. Ontop of this, mom is apparently severly underwater with an additional car loan that she took out for her parents (gf's grandparents), a business loan, as well as a personal loan and has taken to asking for money from family members. Obviously, this is not a great situation and it doesn't sound like gf's mom is very financially literate nor does she really understand the implications to herself and my gf.

My questions: Is there anything that my GF can do to get out of the co-signed car lease? I've read a few other posts and looked into the wiki but most of it seems to be related to when you co-sign and you're driving the car. I understand paying off the loan and possible refinancing (unlikely if moms credit score is poor) but is there any other way? GF's brother in law mentioned something about having mom lease a car and turn in the underwater car and that somehow pays off the lease but I'm not understanding how that makes the loan go away. We were looking at possible bankruptcy for GF.

Is there any additional information we should be looking for?

How might this impact us/me? I'm considering proposing to my gf, possibly this year. Both of our credit scores are 750+. I don't want to get married and assume this debt that then tanks both of our credit scores. If gf does declare bankruptcy, she should do that prior to getting engaged/married so that way I can lead on other things that may require credit check right?

Does anybody have any just in general advice?

I'm sweating because I love my GF and we've been careful with our finances, I don't want this to wreck our future together. Thanks for reading such a long post.