It was already on fire. Clear to see
Classic out of context video. We don’t know whether she set the fire to the left and was starting another one on the right or someone else set Wendy’s on fire and she wanted to join in.
Depends on when the video was shot as to what actually happened. The first few seconds do show what appears to be flames shooting out of the roof but also looks like she is spraying something into it and then you see an orange glow appear.