Hallow, welcome to HCM!
Any intercourse during the menstrual period will not cause pregnancy
as during this phase the egg release does not take place. However, it is not advisable to have sex during menstrual period for the great risk of infection. Blood is a very good medium for bacteria to grow. Such infection may affect her fertility
, may cause pain in abdomen and back due to congestion or at times even may endanger her life.
The egg release takes place 14 days prior to next expected menses. Life of the egg is 24 hours while the sperms are active for 72 hours. Hence a week's period around that day is fertile window. Therefore sexual intercourse on 5th day of menses is very unlikely to cause any pregnancy.
She should get her menses on time. If she does not, you may get urine pregnancy test and/or ultrasonography
performed 8-10 days after missed period as before that these tests are inconclusive.
I hope you got proper guidance.