A mass Jumuah was recently held.Another sister refused to attend the Jumuah salaah as she says that women cannot perform Salaah behind men in Jamaat

There are two apects to your query, a) The permissibility of women's Salaat behind a man, b) The permissibility for women to attend such a gathering.

Even though it is not necessary for women to read Jumu'ah Salat, but if it is read, then this will suffice for the women if the Imaam makes intention for them. (Ielaa-us-sunan)

Due to the corruption prevalent today, it will not be appropriate for women to attend such mass gatherings. Hadhrat Aaisha (Radhiallaahu Anha), the blessed wife of Rasulullah (Sallallaahu Alayhi Wasallam), said after the demise of the prophet Muhammad (Sallallaahu Alayhi Wasallam), 'If our prophet was present today (meaning her time), then he would have forbade women from going to the Musjid because of the corruption.' This was in her time so we can well imagine if our prophet was present today, what would he say.

and Allah Ta'ala Knows Best

Mufti Ebrahim Desai