I talked with the guy on the phone and in a minute he offered to settle for $2300
This is because:
This card doesn't even appear on her credit report
she stopped paying in 2006
Basically, a bad debt buyer has purchased the debt and they know it will be difficult to compel your fiancé to pay. They purchased the debt for a fraction if the $3500 so $2300 would be a huge win for that collector.
Now, they can still sue her. The likelihood of this will be based on the balance (they will TRY to sue for the $15K), the age relative to the applicable statute of limitations, and whether they believe your fiancé has the ability to satisfy a judgement (income, liquid assets, etc).
Here's what you should do.
1.) request a validation letter. This will serve as proof that they own the debt and buy you some time.
2.) find out what the applicable statute of limitations is. This will be based on the state that she lived in when the card was opened. If it is outside of the SOL, they have nothing with which to compel you to pay. They can still call/mail letters until you give them a cease and desist letter.
If you want to negotiate with them, this will give you a basis on which to do so. If you settle with them, make sure you keep a copy of the check and get a letter from the stating the debt as been settled. This, along with your validation letter will be helpful if they "accidentally" sell the settled debt to another entity.