If she had Strangles a month ago, it is possible though not highly likely, that she is still shedding the bacteria. It is also possible that she could become a silent carrier and shed the bacteria at random intervals for years (only about a 25% chance of this). This second situation is why farms which have had Strangles in years past may see it again without having any new animals come onto the property ;)
After contracting Strangles, most horses develop a strong and long-lasting immunity to the bacteria that cause this disease so it's much less likely that they will contract it again.
If you purchase this horse, I would be sure to quarantine her for at least 3 weeks just to help insure that you don't end up exposing any of your other horses.