First of all, having sex within 5 days after ending period is very much safe but using condom has near about 14% failure rate. Though, it depends on fertile period or ovulation time.
On the other hand, 1-7 days period date alteration is normal thing and mostly occurs due to hormonal
imbalance and stress/ mental upset.
Therefore, I suggest her to wait at least 7 days to get normal period and if she does not get it, then she must undergo one home pregnancy
test initially and later one blood test
to confirm or exclude pregnancy.
Following test reports, consult with your doctor for next line of management.
In the mean time, she must take balanced diet, avoid stress, drink plenty water/ fluid, keep bowel habit regular, do yoga/ light physical exercise, eat carotene
based vegetables and take sound sleep.
Be well & always practice safe sex.