I live in Pennsylvania and my finance has a 12 year old daughter and her mom dropped her off to my finance dad for the past 2 months and never came back to get her and my finance dad called and asked him to come and get her so she could get ruled in school.
Is that abandonment?

No but he can go file for full custody.It will not take away her rights though

There is a total misconception about abandonment from a lot of people. Child Abandonment is leaving a minor child without ANY adult supervision.

However, abandonement can be used as grounds to get custody changed or rights terminated.

If you fiancée wants custody, then file for custody. Explain that the mom just dropped the child off and that's why you want a change of custody.