You can't open 2 cap one cards at the same time. The second was denied because you have to wait 6 months between opening accounts. You should have opened the QS instead of the platinum so that you could get rewards. You might be able to message them and ask them to do a PC to the quicksilver in a couple months.
You can try the chase pre qual to see what comes up for her.
Did you ever get true fico scores? Try a $1 trial on CCT. Something seems off if she has 2 accounts open 3 years with no lates and no derogs, her scores should not be low 600's.
@HeavenOhio wrote:CapOne says that the Quicksilver is for those with "excellent" credit. Without the benefit of knowing her score, it's just as well that the OP's fiancée didn't apply for that card at this point.
The Platinum and QuicksilverOne are for "average" credit. While QuicksilverOne offers rewards, people have found it much more difficult than Platinum to eventually product change into Quicksilver. I'd say that under the circumstances, the Platinum is a fine way to get going. The future Mrs. Tyler245 can look into upgrading the card in a few months.
As far as Credit Karma odds, take them with a grain of salt.
She didn't get denied for "bad credit" If she got a $2k limit on platinum, she could have gotten the quicksilver, but you have to wait 6 months between opening accounts, which is why I said she should have went for the quicksilver first because it has rewards and the platinum does not.
There are plenty of people with low scores or rebuilding who have started out with a $300 CL on quicksilver, so it's not really a card for people with "excellent" credit even though cap one says that it is.