A six year old shouldn't be sleeping with either parent. Especially her father at this age. I would just have her dad really drive the point home that he has a special place for her to sleep, if he has a spare room allow her to help decorate it. Even her sleeping in his bed and he sleeps elswhere, explain to your daughter that she is much more grown up now and needs to sleep by herself. I realize that it is a security thing when it has gone on this long for both parents. I understand co sleeping when they are little too to a degree, neither of my children has slept in my bed on a regular basis however. It is a special treat for my kids to get to sleep with me, 3 and 6, they alternate one weekend night to have a sleepover in mom's bed or if they are really sick, but that is the ONLY TIME. My son actually loves his bed, blankie and stuffed animals so much he prefers in his own bed and we all get better sleep in the long run.
I know a lot of people think it is okay and I respect their take on that, however after the age of 2 it is causing way too much dependency when a child should be learning independence after that point. I think it becomes a security blanket for the child that is a hard habit to break later, parents even single ones like me need their space and sleeping should be one of them. A child needs to learn to sleep alone or then sleepovers, traveling and issues are a problem later on and can hurt them socially. I loved the Dr Phil episode on this one, even Dr Sears who advocates cosleeping said there is a limit to the age it is okay anymore. Girls need to learn to be discreet and seek privacy even with their fathers, not dressing in front of them, shutting the door in the bathroom, etc. It is just teaching them that now it is time to respect themselves and their privacy.