Don't think it's a sidecar under that sheet, there's no nose on it, just something high up. Any sidecar even with a screen would have an outline that was nose first surely ?
The frame looks to be a light colour, quite unusual if standard . Can only think of light blue, green or beige ?
Bsa or Triumph twin is my guess, looks like he needs a longer clutch cable though !
edit to add - single downtube rules out Bsa twin, could be anything !