No, I don't think a UTI would have anything to do with her period being late. She really should see a Dr about that and to find out why her periods are so irregular. This can happen in teen girls, and even grown women. Some Drs will want to put them on birth control pills to get their periods regulated. In my opinion, this would be a good thing to help with an unplanned pregnancy and her irregular periods. Does she have someone that can take her to the Dr? Her mom, an aunt, a friend?
My girlfriend also thinks she has a UTI but I doubt that has anything to do with the menstrual cycle, but you never know.
You say she has irregular periods so that could be what's going on. Of course if you're having unprotected sex, which you did the one time, she could very well be pregnant. Pre semen can get a girl pregnant so the pull out method is not a form of birth control. All you can really do right now is wait to see if she gets her next period. Or, remember when you had unprotected sex and test a couple of weeks after that, if she has'nt gotten her period.