Yes, this happened to me in my first pregnancy. I lost the twin at around 9-10 weeks...it is called "vanishing twin". I bled until week 17, somtimes very heavily and it was very very disconcerting. I did lots of googling and was suprised to learn that it is estimated that only 30% of twin pregnancies results in twin births. It helped me realize that it was not uncommon. Eventually the miscarried twin was reabsorbed back into my body, but I admit there was a definite twinge every time I had an ultrasound to see where the previous twin had been and see the remnants of the twin. By the last few months of pregnancy it was completely gone. Emotionally it was a struggle too as you mourn the miscarried baby and are still thrilled and terrified for the one remaining.
I was sure with every episode of bleeding that I had miscarried the other one. They put me on low activity, though not bedrest, and I had to wait and see. The doctors said it was up to a higher power at that point. I was 40 at delivery as well. Ultimately, I had a wonderful happy healthy baby boy...and am pregnant again with a single this time.
It is not uncommon, but it is very stressful for the mommy to be.
Good luck to your friend. I remember the emotions of it all very well.