I agree with other posters who made the point of:
If they were travelling via train would the OP expect her friend to pay for her train ticket as well as her own?
If the event had come at a personal, monetary cost to the OP then yes, the friend should have paid the petrol costs.
But the OP didn’t have to pay out for anything yet her friend is £40 down.
If I was the OP, I would never expect a friend of mine to pay for all the petrol costs just because I’d invited her along to a free weekend trip that I’d been gifted.
I haven’t seen OP say anything that implied she’d worked extra hard, or completed a special task at work or that she’d gone the extra mile which meant she’d been rewarded with the trip, which is what some posters seem to be implying. All OP said is that the trip was a “perk of the job”.
And even if OP’s trip had been a reward for her hard work, I still think it’s really wrong to not pay for her own half of the travel costs and expect her friend to pay for it all.
It’s intriguing how split the responses are as usually most AIBU opinions tend to heavily sway one way or the other.
I suppose it just demonstrates that we all have different views on what friendship is depending on whether we are on the OP’s side or the friend’s side.