Adding her will certainly boost her score as long as AMEX reports AU accts, most notably, to TU. You can ask AMEX to (please!) do an acct update to the 3! The cra's only report what the accts send, so asking TU or the others is fruitless.
How much will this raise her score? Hard to say. She should have at least one cc acct posting a small balance to show a real util%. If she's got a 0 balance across the board...0% of whatever limit is still 0. Even a buck posted will activate a real util% number.
Good luck!
Actually, the statement cut date being the date they report is the "norm". Don't take my word...ask around. Folks here will tell you.
I am an AU on one of my mother's cards. We had to specifically request that the card be reported on my credit report, and was asked for my SS number at that time.
Just having this card appear on your wife's credit report as an AU will not do much for her FICO, unless it affects her average age of accounts positively, and/or she has a current high utilization on the cards she has reporting now and therefore adding this one with 0 balance will change her utilization %.
Simply adding another card will not affect her score if she already has <9% utilization on her current cards since credit limits don't matter, it's the utilization % that calculates into FICO.
In my case, adding my mother's card increased my average age of accounts from 1 year to 9 years so it increased my score significantly. Utilization remained the same since we both pay our cards in full so they show $0 balance, which is something we need to change because one of the things hurting my score is that it appears I'm not 'using' credit since they are reporting $0 balances. Keeping a balance so the utilization is <=9% will show that I am using credit responsibly and will increase my score more than if I always paid in full each month.
Starting Score: 524