I am new on building/rebuilding credit, so this should be taken as second opinion, not as expert advice. I would go ahead and do the Amex, and then sit tight for a year other than perhaps requesting CLIs. The reason I say that is, that would be three cards, and getting in with Amex at an early age is a good thing IMO. I wouldn't worry about utilization much at this point as long as she doesn't start carrying balances and paying a lot of interest. She's off to a great start (better than my current situation), good luck!
Has she tried BB&T? they have approval on soft pull. I would try, i mean what is the worst that can happen since it is a soft pull anyway.