public class GroupData : ObservableCollection<DetailData>
{
    public string Name { get; private set; }
    public GroupData(string name, ObservableCollection<DetailData> d) : base(d) 
              { Name = name; }
}

public class DetailData
{
    public int Nr { get; set; }
    public string Detail { get; set; }
}

public ObservableCollection<GroupData> GetData()
{
    ObservableCollection<GroupData> data = new ObservableCollection<GroupData>();

    data.Add(new GroupData("Group1", new ObservableCollection<DetailData>
    {
        new DetailData { Nr = 1, Detail = "Test 1" },
        new DetailData { Nr = 2, Detail = "Test 2" }
    }));

    data.Add(new GroupData("Group2", new ObservableCollection<DetailData>
    {
        new DetailData { Nr = 3, Detail = "Test 3" },
        new DetailData { Nr = 4, Detail = "Test 4" }
    }));

    return data;
}

[RelayCommand]
private async Task ButtonOk(DetailData data)
{
    foreach (var item in GroupData)
    {
        foreach (var child in item)
        {
            if (child.Nr == data.Nr)
            {
                item.Remove(child);
                break;
            }
        }
    }
}

<RefreshView Command="{Binding RefreshCommand}" IsRefreshing="{Binding IsBusy, Mode=TwoWay}">
    <ScrollView>
        <Grid>
            <ListView ItemsSource="{Binding GroupData}" 
                      IsGroupingEnabled="True"
                      HasUnevenRows="True">
                <ListView.GroupHeaderTemplate>
                    <DataTemplate x:DataType="daten:GroupData">
                        <ViewCell>
                            <Label Text="{Binding Name}"/>
                        </ViewCell>
                    </DataTemplate>
                </ListView.GroupHeaderTemplate>
                <ListView.ItemTemplate>
                    <DataTemplate x:DataType="daten:DetailData">
                        <ViewCell>
                            <Grid >
                                <Label Text="{Binding Detail}"/>
                                <ImageButton Source="ok.jfif"
                                    Command="{Binding Source={RelativeSource AncestorType={x:Type local:MainPageViewModel}}, 
                                    Path=ButtonOkCommand}" 
                                    CommandParameter="{Binding .}"/>
                            </Grid>
                        </ViewCell>
                    </DataTemplate>
                </ListView.ItemTemplate>
            </ListView>
        </Grid>
    </ScrollView>
</RefreshView>

<RefreshView >
    <ScrollView>