What Are The Chances Of Getting Pregnant Without Ejaculation?

my daughter had unprotected sex around October 5. She actually ended up getting scared and she asked him to stop, in which he did. he did not ejaculate inside her. she is very irregular with her periods and usually only has one every two or so months. her last period was sept 22 approx. we did a pregnancy test on the 16th of this month and it was negative. should i be worried?