C# (as usually short - 1 line - and elegant, no indexing):

public static string bobAndBen(List<List<int>> trees)
{
    return trees.Aggregate(0,(xor,t)=>xor^(t[0]==0 || t[0]==2 ? 0 : (t[0]-1)%2+1))==0
    ? "BEN" : "BOB";
}

Python must be 1-ne as well........