Be casual friends with her for now. Try extremely hard not to let your feelings for her go over the line. Don’t ever slip up and be lovey-dovey, don’t get touchy feely with her act jealous or try to get her to be more interested in you than she currently is. Do not text her after 9:00 pm at night (or call her or e-mail her) and when you do contact her, keep it casual and short and try to refrain from discussing how you feel about her. You can and probably should ask her if she is exclusive with her boyfriend and what her plans are with him when he gets back.
She is likely to say that she is exclusive with him and that they plan to continue to date when she gets back. If that is the case, just let her be, but you could let her know that if anything changes in the future, you would be interested in dating her, but not unless or until she is broken up, and that you have no desire to interfere with their relationship. Then don’t interfere! That means, be casual friends, but avoid talking about deeply personal things with her, because if you do, she might become very attached to you and either cheat on her boyfriend, or feel torn between the two of you. Don’t put her in that position.
And don’t put her boyfriend in that position too. It’s very uncool. If they end up breaking up, over the summer or when he comes back, give her a little time and then let her know that you are still interested (if you still are by then) but don’t stop getting to know other girls that may be of interest to you, give them a chance.
Be one of the good guys and don’t end up being one of these douche-bags that a lot of young women have to endure as their very first boyfriend. You have no idea how many hours us jellies have spent trying to console those people. You’ll be doing some lucky lady a huge favor!