To detect when a general help dialog is closed, add the following callback to the dialog:
XtAddCallback (helpDialog, DtNcloseCallback, HelpCloseCB, (XtPointer)NULL);
Where helpDialog is the widget ID for the help dialog and HelpCloseCB is the name of the callback procedure you've written to handle closing dialogs.
To detect when a quick help dialog is closed, add the following callback to the dialog's OK button:
XtAddCallback (DtHelpQuickDialogGetChild (helpDialog, DtHELP_QUICK_OK_BUTTON), XmNactivateCallback, HelpCloseCB, (XtPointer)NULL);
Where helpDialog is the widget ID for the help dialog and HelpCloseCB is the name of the callback procedure you've written to handle closing dialogs.