I agree with Web that some therapy sounds in order. The earlier it starts the better the outcome. My other suggestion would be you and your partner letting her know that it is okay for her to share her feelings about missing her mom or any other feelings related to her parents with you guys. And be very blunt about it so she doesn't have to guess the meaning of what you are saying. Let her know you wont be angry at her no matter what she shares. The second part of this is then being able to stay calm when she shares things that aren't easy to hear (anger, rage, fear etc are possibilities). Since she seems to be hating school this year, checking in on how things are going at school with her main teacher is important. Find out how things are going academically and socially. There might be something that is happening at school (bullying, trouble with academics etc) that she isn't sharing with you two.

It is wonderful that you are so involved in her life and concerned about her. She is likely to have such concerned parents. As you said that dad doesn't understand 10 yr old girls very well (not surprising especially if he didn't have sisters) have her teach him something that she enjoys doing. It will strengthen their bond and let him learn more about what life is like through her eyes. I hope things work out.