My friend has just come back from Berlin, she had a ball.
Go with your daughter and her friend and make special memories for her 50ieth, it will be nice to be a girls threesome. It may well be your daughter wishes to do this as a gesture of thanks to her friend for supporting her, perhaps this friend hasn't got anyone to go on city breaks with. I am 62 and went to Venice with my 23 year old daughter, I didn't really have the energy to do everything she suggested, but felt I had to as we were only there for a few days. Having a friend along would have meant I could have rested more if they'd gone off together, the bridges killed my calves! As for your DH, you've said he isn't into holidays like this, so your DD knows this too.
I hope you decide to go, my daughter has been and loved it too.