A hefsek taharah must be performed before the clean days can begin. Noticing that bleeding has ceased cannot substitute for performing a hefsek taharah. Even in this case, where you are returning from a journey, your wife's clean days and immersion are delayed by one day.

There might be room for exception if your wife checked herself internally at some earlier point after she stopped bleeding. Please get back to us with details if that is the case.

If similar situations arise in the future, we recommend that your wife do a hefsek taharah and count her seven clean days as she normally would, at the end of her period. She could then delay immersion until a convenient night shortly before you return. (After completing the seven days, she would not need to wear white or do any additional bedikot.)