Without more information, it is impossible for me to tell whether the woman was offering Mass or whether she was leading a Communion service. Given the explanation offered you, I am inclined to give the benefit of the doubt and think that it was a Communion service.
Laypeople (including laywomen) are allowed to lead Communion services in the absence of priests. The Eucharist is not consecrated at this service – something only a priest can do – but is consecrated beforehand at another Mass. For an annual example of the difference between Masses and Communion services, remember what happens on Holy Thursday and Good Friday. Because Masses are not offered on Good Friday, the priest consecrates double the usual amount of hosts on Holy Thursday and the extras are reserved for the Communion service on Good Friday.
If you stayed for the entire service and actually observed this woman attempting to consecrate the bread and wine, then I recommend that you contact the bishop of the diocese in which this church resides and report to him what you witnessed.