GWR▸ may have been unaware of this. The news story states that it was the station manager, a Southern employee, who would not let her on to the station.
Maybe.
If GWR made / agreed with the decision ahead of time, that's one thing.
If GWR were made aware of a decision taken by Southern, properly done on genuine safety grounds, the so be it.
If Southern made a unilateral decision for what even reason, and didn't even have the politeness to inform GWR ... let's say that if I was GWR in that circumstance, I would be pretty miffed!