Period = no risk of pregnancy?

Yes, she could. As well, the same -- and sometimes slightly higher -- STI⁠ (STI: Sexually transmitted infections: illness, infection and/or disease which is often or can be transmitted through sexual or other intimate contact, like HIV, Chlamydia or Herpes. Some people call STIs STDs.) risks are present during menstruation⁠ (menstruation: The shedding of the uterine lining as part of the menstrual cycle.) as any other time.

For most people who can become pregnant, pregnancy⁠ (pregnancy: The state of carrying a developing embryo or fetus within the uterus. Medically, someone is considered to be pregnant when an egg has been fertilized by sperm, cells divide, and the fertilized egg is implanted within the lining of the uterus.) is substantially less likely from intercourse⁠ (intercourse: When people interlock their genitals and move together as feels good to them for the purpose of sexual stimulation and/or reproduction.) that happens during menses⁠ (menses: The flow during menstruation, composed mostly of endometrium, including blood. Sometimes flow contains portions of the endometrium as solid tissue, which is often confused with blood clots.) than from intercourse at other times. However, since sperm⁠ (sperm: Reproductive cells of people with penises which can fertilize an ovum and create pregnancy.) can live in the vagina⁠ (vagina: The internal passage leading from the opening of the vulva to the cervix of the uterus.) for several days, and some people ovulate early in their cycles, for some people, unprotected sex⁠ (sex: Different things people choose to do to actively express or enact sexuality and sexual feelings; often this involves genitals, but not always. ) during menses -- especially at the very end of it -- can be just as much of a pregnancy risk as during other times of the fertility⁠ (fertility: The ability to become pregnant or other things having to do with becoming pregnant (eg, “the fertility clinic”).) cycle.

So, if this happened less than 120 hours ago, and your partner⁠ (partner: In a sexual context, a person with whom someone is having some kind of sex. The term "partner" can be used for all kinds of relationships, not just serious ones. "Partner" can also mean the person someone is with in a romantic or familial partnership.) does NOT want to become pregnant, we'd advise she obtain emergency contraception as soon as possible.

And in the future, I strongly urge you not to gamble with pregnancy and STI risks, especially considering how easy it is to be more responsible about sex. It's very easy to keep condoms handy at all times and always use them, and it's also easy to just stick to outercourse and other activities -- things like mutual masturbation⁠ (mutual masturbation: When sexual partners masturbate together. Sometimes people also use "mutual masturbation" to mean manual sex (fingering or handjobs) done at the same time.) , dry sex⁠ (dry sex: A kind of sex where people have clothes on, but are pressing their genitals together. This can also be called frottage, grinding or dry humping.) , even manual sex⁠ (manual sex: Sometimes also called digital sex. Kinds of sex involving the hands and fingers to sexually stimulate the genitals or other parts of the body. Fingering, handjobs or "fisting" (deep manual sex) are some kinds of manual sex.) -- which don't present these risks if you want to be sexually active⁠ (sexually active: Someone who has had or is currently having some kind of genital sex with a partner or partners.) when you don't have latex barriers around.