Wedding ceremony no windows low light- flash help?

I’m shooting a wedding in a community centre with no windows and the bride wants the lights off because they’re the bright fluorescents. She said her DJ is bringing “some lights.” I’ve tried to contact the DJ to see exactly what he’s bringing but I have not used off camera flash ever and I only use flash on camera for ceremonies that have ambient lighting. Stuck at what to do and how to prepare because I don’t know how dark it will be and I think she’s expecting the DJ to bring ALL the lighting??