How do I know the debt is valid or not?

My client received a letter from National Action Financial Service and it says that she owes the total of $50,000 from 2000. She did not know about this debt and she thinks that it's a very old debt from her ex-husband who filed bankrupcy in 2004. So as a translator, I helped my client to call them and ask them about the debt. They said the debt was under her name but not her exhunsband. They offered few settlements to settle the debts (from $8500-$4500). As I doubted it was fraud, I googled this company and people had bad comments about it.
Now this company keeps calling me (who is a 3rd party) to collect the debt and pass messages to my clients
How can I find out if the debt is real or not? And how can I stop them from calling me? Are they legal to call me all the time? - Is this your question? Add additional information