<Window x:Class="TurCC.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:c1="http://schemas.componentone.com/winfx/2006/xaml">
<c1:C1FlexGrid
    Name="gr_CertUsers" AllowSorting="True" AutoGenerateColumns="False"
    SelectionMode="Row" AllowDragging="None"
    GridLinesVisibility="All" AllowDrop="False"
    HeadersVisibility="Column"
    ItemsSource="{Binding Path=CertUsersCV}"
    SelectedItem="{Binding Path=SelectedCertUser, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
    CellEditEnded="gr_CertUsers_CellEditEnded"
    PrepareCellForEdit="gr_CertUsers_PrepareCellForEdit">
    <c1:C1FlexGrid.Columns>
        <c1:Column ColumnName="UserLogin" Header="Логин"
            Binding="{Binding Login, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">
        </c1:Column>
        <c1:Column ColumnName="UserPassword" Header="Пароль"
            Binding="{Binding Path=Password, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">
        </c1:Column>
    </c1:C1FlexGrid.Columns>
</c1:C1FlexGrid>
</Window>


private void gr_CertUsers_PrepareCellForEdit(object sender, CellEditEventArgs e)
{
    // думаю, здесь можно вставить в ячейку реальный пароль
}

private void gr_CertUsers_CellEditEnded(object sender, CellEditEventArgs e)
{
    //думаю, здесь можно вернуть звёздочки для ячейки, вместо реального пароля
}

private void gr_CertUsers_CellEditEnded(object sender, CellEditEventArgs e)
{
    var vm = this.DataContext as TurCCVM;
    Column col = (sender as C1FlexGrid).Columns[e.Column] as Column;
    if (vm != null && vm.SelectedCertUser != null && col.ColumnName == "UserPassword")
    {
        vm.SelectedCertUser._bShowPswrd = false;
    }
}

private void gr_CertUsers_PrepareCellForEdit(object sender, CellEditEventArgs e)
{
    var vm = this.DataContext as TurCCVM;
    Column col = (sender as C1FlexGrid).Columns[e.Column] as Column;
    if (vm != null && vm.SelectedCertUser != null && col.ColumnName == "UserPassword")
    {
        vm.SelectedCertUser._bShowPswrd = true;
        var v = vm.SelectedCertUser.Password;
        vm.SelectedCertUser.Password = null;
        vm.SelectedCertUser.Password = v;
    }
}

public class UserRights : ObservableObject
{
    public string _UserName;
    public string _UserPswd;
    public bool _bShowPswrd = false;
    #region Properties
    public string Login
    {
        get { return _UserName; }
        set
        {
            _UserName = value;
            RaisePropertyChanged("Login");
        }
    }
    public string Password
    {
        get { return _bShowPswrd ? _UserPswd : "*******"; }
        set
        {
            _UserPswd = value;
            RaisePropertyChanged("Password");
        }
    }
    #endregion Properties
}