This is a screenshot from the season 5 finale, the wedding scene (Zoom out at the end).
I have a few reasons to believe it's her:
1. We know that besides the usual gang (Sheldon, Leonard, Raj, Penny and Amy) and the happy couple (Howard and Bernadette), the only other guests are Bernadette's parents (Bottom of the screenshot, black suit and pink dress) and Howard's mom (The only person left on the roof).
2. The woman in the red dress is obviously fat (About twice as big as any other person on the roof), as commonly mentioned.
3. At the beginning of the scene, Howard yells to his mother,