Boyfriend over bestfriend? HELP?

I already have plans with my boyfriend that im going out with him on friday.. i only see him once a week so i dont wannt to miss out on this day.. but my bestfriend just called me asking if i could go with her to this university she's planning to get in to when she graduates and that's the only day she can come visit the school.. she wanna get more infos about it.. but i lied and said i am working. i feel really bad for lying, but if i say i cant go on friday because i am seeing my boyfriend she'll get upset cause i am ditching her for my boyfriend.. help ?xx