It was just amazing. Totally unplanned, and her dh was with her, but she called me when her waters broke and asked if I'd come to the hospital. I just stayed with them through contractions, rubbing her back, dealing with midsives, getting him coffee and left them to it as she climbed into the birthing pool just before delivering.
She had a perfect birth, no tearing and left the same day. It was such a contrast to my birth, with just me and a desperately tired and panicked dh trying to grab an overworked midwife's attention. It did make me think it makes total sense to have a third person with the couple in labour.