Get Plan for Foreign Customers

It's unlikely that she can get a plan setup before arriving in the US.  She will need her Student Visa number to get a postpaid plan with T-Mobile.  Most students though, especially international students, go with prepaid.  Either way, she will have to visit a T-Mobile store to get a SIM.  Whether or not it can be paid with a credit or debit card based in Spain depends on your bank.  In a store, it should work.  Online payments is much less likely it will take the card.