Thank you for your question.
While it may be "normal" for some families to do this it is generally not an acceptable practice. I am a man with a daughter and we went through a divorce .I can honestly say that this would not have been normal for me or my daughter at that age.
I believe most men would say this is not healthy emotionally for either person involved.
He may be feeling a serious loss from the divorce (children do also) but it is not a wise move to sleep with the daughter and may need a little help getting through this time. It's a selfish move to insist she still sleep with him or to encourage her by allowing this.
The daughter may feel guilty about the divorce (lots of kids do) and helpless to tell dad NO. Because he is defending this behavior I am reasonably sure that this did not happen before the divorce and knowing teenage girls this is probably NOT her choice.
Divorce makes most people feel a little anxiety and unsure of the "security lack" .
Sometimes father's don't really want "their little girl" to grow up but a 13 yr old in today's world is already well on her way physical to becoming a young lady with wants and needs of her own to establish her life.
Sleeping with daddy is not a prerequisite for this.
Dad needs someone he will listen to so he stops this TODAY and sets himself and his daughter free from this lifestyle.
Regardless of whose choice this is it is a behavior which misleads the daughter and does not help her become a confident teen who can later say NO to the advances of a date. In otherwords he may be "setting her up" without realizing the consequences of what seems so innocent now.
Eventually when he starts dating or remarries this behavior can have other consequences.He really needs to think this through so he can be honest with himself.
If you have the chance to talk with the daughter you may find that she is not seeing any reason why this should not happen. You can let her know that she's going to be a beautiful teenager and she'll want her privacy in her own room. Thoughts like this can help her start her new life with her dad and she will eventually ask for her own room if she doesn't have one already.
She needs to know Dad's are just dad's and they are not going to be the main figure in her life forever. That's a place someone will fill eventually whom she will sleep with perhaps someday.
It's ok for her to want to sleep in her own bed.
If you feel this is a situation where there's more going on than what he's telling you then you may want to let Social Services know about this.
Should you be dating this person you'll want to express your opinion in a kind,diplomatic way,in front of the daughter encouraging her to find new friends in her dad's neighborhood.Perhaps you can find some activity to get her acquainted with others closeby.
I am including a link that you may find helpful in offering help to either the dad or the teen or even just for an understanding for yourself of life after divorce.
An eye opening link concerning teens and divorce:
More help on teens/divorce
Let me know please if you have any questions ? Otherwise just click "accept" and leave a positive feedback.