Fuck me, that's horrific. And I say that as an ex-City person.
I did 5 years in the City (not at an investment bank) and my average week was probably 60 hours, bad weeks 80-100 hours (probably about once a month) and good weeks about 50 hours - never less (until I resigned!). I found it completely manageable but not at all enjoyable, especially as the circle of friends I moved to London with all had much more sociable hours.
But, starting at a top firm in the City opened doors for me and continues to do so. I would support my own DC going to the City, but make sure I found ways to help them - keeping the lines of communication open, and helping them realise that money isn't everything.
I left the City when I felt me attitude turning - I started to become rude and aggressive, and when I snapped at someone I knew it was 100% time for me to leave. I'm glad I had the realisation that it was changing me, and I think that's what you need to watch for in your DC.