I think this link from Publication 519 gives the details about that:

https://www.irs.gov/publications/p519#en_US_2021_publink1000222182

No, filing for more than 7 years does not mean he is required to file US returns forever.  But there is an 8 year rule that could require some additional things, so I suspect that is what your client is referring to.

https://www.irs.gov/publications/p519#en_US_2021_publink1000222182

EDIT: Oh, I had this window open too long and Itonewbie answered it several minutes ago (with a much better answer, as he is highly experienced with this subject).