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I am very sorry to hear about this situation. On this website, I do not always get to give good news, and this is one of these times.
Technically, they can sue you for breach of contract. They will likely sue both of you, if they decide to sue. If they do, and they get a judgment, then they can enforce judgment (although Texas makes it very difficult for judgment creditors to do so since it is a debtor-friendly state).
Ergo, simply stopping payment is never a good idea. Someone in your situation can try to sell the timeshare (even your part, if possible) but only use a reputable agent, or, donate. You will likely need your former friend's cooperation, and hopefully he would agree if he sees that this is in both of your interests - he is as liable as you are for a judgment.
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