I think the big problems comes from the fact that an independent shop may never be able to prove it to the satisfaction of the first shop, who have lots of incentive to stonewall until Tina runs out of cash.

Even if the timing belt isn't installed properly, who's to say that it didn't jump a few teeth after her valve guides failed and caused internal engine damage?

Another way to ask the question: Does she want to be right, or does she want to be happy? Even if she's right about the first shop making the mistake, the cost of proving it and pursuing any damages would likely make her even more unhappy. Not a good solution for her.

I wish I could cast some less dreary light on this, but I was born a pessimist. It's just easier to move on, and never use that shop again if she really suspects they made a mistake.