should She tell someone?

ok, so i work in a fast paced environment, we had to hire some temps to help us out because this project that they had me do, i needed some help with. well this morning one of the girls that has been helping me comes in and tells me that she got a job offer, i was really excited for her because it sounded like a nice opportunity. So then i asked her when she was going to let our boss know (mainley so i knew when somone else would be here to help me) and she told me that she isn't telling anyone she is just going to stop showing up. am i being selfish because i know i need help, or should she tell someone that she is leaving on the 18th? please help!