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The date that she acknowledged in writing that she owed this debt is going to be crucial -- especially if she said she intended to repay it. The writing doesn't need to be signed, e-mail will work
When was that?
I am sorry to have to tell you this, but the statute of limitations for breach of contract in Colorado is three years from the breach. The time to file a lawsuit would have been no later than May 31, 2006.
If this acknowledged the debt later and promised to pay it again (for example, by sending you an e-mail that said, "I'm sorry I haven't paid you back, I promise I will do so as soon as I can), that could restart the statute of limitations from the date the later promise was made. For example, if you got this kind of e-mail on January 1, 2008, the statute would not run until January 1, 2011.
Here is more information about statutes of limitations in Colorado: http://www.expertlaw.com/library/limitations_by_state/Colorado.html
I wish I had better news for you.