I believe the normal definition used for a thin credit file in most scoring models is less than 3 trade lines.
The importance is the scoring category, or "bracket," that will be used for the subsequent scoring.
I dont believe that his file is categorized as "thin."
Separate but related is mix of credit. With both revolving and installment, he has a mix. FICO likes to see multiple revolving, as heavier emphasis is placed on showing of effective use of discretionary credit. So one more revolving would, in my opinion, be helpful.
AU accounts can produce a score boost, but if the creditor does a manual review of his credit report, it may rasie doubt as to the significance of his score should they desire to see his risk based only on his own credit history. Often not done for lower CL cards, which often base decisions primarily upon score without the additional time and expense of doing a more thorough manual review, so a good way to rebuild. They have no way to "back out" the history of an AU account to produce a score based only on his own history, which could become a factor for higher credit decision making.
The number of accounts, particularly revolving, has secondary implications in other categories as well. Since FICO also scores such factors as % with balancem with one revolving, he is always at 0% with balance or 100% with balance. Addition of a second would permit 0%, 50%, or 100%. And so one.
But addition of new accounts can also have a short-term offset by addition of inquires and reduction of AAoA. Which rasies questions of how long until the score will actually be used to app for new credit. If a year or more in the future, the new inq impact will be gone.
It gets complicated. Single actions usually have multiple effects.
