Well, yes, a late period is a sign of pregnancy. At this point, a home pregnancy test - that you can get at any drug or grocery store would be accurate.
As RockRose mentioned, that is definitely a sign and, in fact, is usually the first sign that a woman notices when she is pregnant. However, this would only be a sign if you & she had had unprotected intercourse. The odds of pregnancy are zero unless she has engaged in an activity that can cause pregnancy.