It wasn't cancelled early, it was cancelled at 21:46 on a scheduled departure time of 20:20, though passengers were probably told a little before that. There is currently a 6 hour time difference on the sourcing there. The gate opened at 20:41, and they had about 20-30 minutes to get everyone on board, but in the end they were given a slot time of 21:36, which would be too late for MUC. I would imagine your friend would have been aware of what was going on here, if it's at the gate then usually this does get explained in the Tannoy announcement. But by the looks of it the ABZ-LHR service was badly delayed, G-EUPU only got to gate 2 at 20:52 with the last bag off at 21:33. So this was unlikely to work out, the ABZ service was just too late to permit a turnaround sensible for MUC. It will be OPEY.

There is an argument that this should have been cancelled when it was clear that ABZ-LHR could not get to a T5 gate before 20:15, the maths are fairly clear here.