Rule of thumb for going AU: the account should be:
1. older than your AAoA (average age of accounts)
2. immaculately clean, no baddies anywhere
3. always paid in full, or with an occasional minimal balance that is promptly paid off
4. reported to all three credit bureaus (not all AU cards report)
If his card meets these criteria, then yes, it should help.
One personal comment about going AU for the sole purpose of helping your credit: I'm AU on my husband's Discover card, to get some additional history. (I was a "housewife at home" for a long, long time, with no credit in my name.) I love my card --it's very pretty; the Monet "Nymphaea" design --but I never use it. It sits in my drawer, in the dark.
IMO, if someone is generous enough to offer to help someone build/ rebuild their credit, the ethical thing to do is to not use the card and potentially cause problems for the actual cardholder. Like I said, just my take on it.
* Credit is a wonderful servant, but a terrible master. * Who's the boss --you or your credit?
FICO's: EQ 781 - TU 793 - EX 779 (from PSECU) - Done credit hunting; having fun with credit gardening. - EQ 590 on 5/14/2007