Is it a good idea to pay past debts?

I found your website and hope that you might be able to answer this question. I am advising a friend regarding old debts that are 3-5 years old. He came to me after he was sued by a the law firm representing the credit card issuer for the outstanding balance. He paid the entire balance and accrued interest, but during our discussions, he said that he has other bills that are outstanding and as he has become financially more stable would like to make payments to take care of these balances. He has asked me what the impact of such payments and paying these balances off (a couple he already took care of through reduced settlements), but still has about three more accounts to go. So here are my questions:

1. Is it ever a good idea to settle an account for less than what is owed?

2. How long will a debt settlement impact his credit assuming that the original debt is about 3-5 years old, but the settlements occured recently with debt collectors?

3. Should he continue to pay the previous debt?

4. What impact will paying the previous debt have on his credit score, in general?

5. Should he be working with the debt collectors or the original debt issuer?

6. Can he require the debt collector to report the timely repayments under any new arrangement to be reported to the credit bureau?

Any other advice that you can give on the subject is greatly appreciated!

To add additional context to the situation, the debts were incurred while living in South Carolina, but he has since re-located back to Michigan.