I advise going through completely separately and not meeting up until you've passed through Customs and are outside the CBP zone. She should not mention that she's in the US to see her American boyfriend, and you should not mention that you're bringing a girl with you. The general assumption is that she is coming to stay and will not return to Costa Rica if she's staying on your dime. She needs to be prepared to show that she has her return ticket, that she has money or credit to stay in the US for her vacation, that she has a job, family and house back home that she needs to return to.