Do you anticipate your client to have large balances due on those returns?  Does your client have any foreign-related items such as financial accounts, assets, CFC, trusts, etc. for which information returns would have been required?

If neither of these apply to your client, the IRS would normally settle for 3 to 5 years.  Once you have a better handle of the facts, you could call the PPS with a PoA and discuss the filing with them.

In the event your client has foreign financial assets for which information returns have not been properly filed, you may want to check whether your client is eligible for the streamlined procedures.