Me in the position of the driver: I'd be disappointed if my friend didn't offer to go in on the fine with me, but I wouldn't directly ask.
Why? As the driver, it is you who decides where you park, not any passenger with you. If you went to the venue alone you'd be in the exact same position. Why would you even think to expect someone would offer to split the bill in this circumstance?
If the passenger had made the decision you should park there, then that's a different matter I guess, but otherwise, just accept the fine,or politely respond to the PCN and hope for the best. I did this recently with a Southampton council PCN and got it revoked.
I went to London a while back and got a PCN for parking somewhere without realising that area of London is no parking without a permit (no visible signs on road). My friend did offer to pay a bit but I refused the offer and thought nothing more of it.
Just be cool.