Hello mmorozov,

In the RecordActivated event(or any event as long as there is an active row) you can get the cellValuePresenter from a cell using the static method off the CellValuePresenter class.  Once you have the CellValuePresenter, you can use the VisualTreeHelper.GetChild() and GetChildrenCount methods to find the child that matches your criteria which is based on the name and type of object.  I have pasted some code below.

private void xamDataGrid1_RecordActivated(object sender, Infragistics.Windows.DataPresenter.Events.RecordActivatedEventArgs e)
        {
            CellValuePresenter cvp = CellValuePresenter.FromCell(((DataRecord)e.Record).Cells[0]);
            DependencyObject dobj = FindChild(cvp,"textModified",typeof(TextBox));
            if (dobj != null)
            {
                TextBox tb = (TextBox)dobj;
                tb.Text = "I found it";
            }

        }

        public DependencyObject FindChild(DependencyObject reference, string childName, Type childType)
        {
            DependencyObject foundChild = null;
            if (reference != null)
            {
                int childrenCount = VisualTreeHelper.GetChildrenCount(reference);
                for (int i = 0; i < childrenCount; i++)
                {
                    var child = VisualTreeHelper.GetChild(reference, i);
                    // If the child is not of the request child type child
                    if (child.GetType() != childType)
                    {
                        // recursively drill down the tree
                        foundChild = FindChild(child, childName, childType);
                    }
                    else if (!string.IsNullOrEmpty(childName))
                    {
                        var frameworkElement = child as FrameworkElement;
                        // If the child's name is set for search
                        if (frameworkElement != null && frameworkElement.Name == childName)
                        {
                            // if the child's name is of the request name
                            foundChild = child;
                            break;
                        }
                    }
                    else
                    {
                        // child element found.
                        foundChild = child;
                        break;
                    }
                }
            }
            return foundChild;
        }

This should do the trick.  Have a good day!  Any problems with this, let me know.