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If for any reason it continues to report. He can with each bureau dispute the AU account as "not mine".
I'm not really understanding if he has 3 open CCs, with higher credit lines, and low utilization, that you would pick a lower line of credit card with late payments to be added to his reports.
He needs to build on what he currently has. Don't know if he has some derogs, but I'm gathering by scores it is a dirty file. Need to clear those things up, build a relationship on what he has, especially if he has a pristine payment history with them, they will hopefully grant him credit line increase in the future.
The balance decrease isn't going to cause a score decrease. My bet is that the decrease came because the card had lates as part of its history.
If you get the lates removed it won't hurt his score, but access to an additional $500 of credit as an au isn't going to help much either.
He has the three cards now so he sould be good with those provided he dosen't have high util, or lates on them.