Outlook – Read another user’s calendar

Calendar delegation is a feature of Exchange, the Graph API and Outlook API do not allow the user to access the delegated calendar.
Currently, the alternative workaround could be use the EWS. And here is an sample for your reference:

static void DelegateAccessSearchWithFilter(ExchangeService service, SearchFilter filter)
{
    // Limit the result set to 10 items.
    ItemView view = new ItemView(10);

    view.PropertySet = new PropertySet(ItemSchema.Subject,
                                       ItemSchema.DateTimeReceived,
                                       EmailMessageSchema.IsRead);

    // Item searches do not support deep traversal.
    view.Traversal = ItemTraversal.Shallow;

    // Define the sort order.
    view.OrderBy.Add(ItemSchema.DateTimeReceived, SortDirection.Descending);

    try
    {
        // Call FindItems to find matching calendar items. 
        // The FindItems parameters must denote the mailbox owner,
        // mailbox, and Calendar folder.
        // This method call results in a FindItem call to EWS.
        FindItemsResults<Item> results = service.FindItems(
        new FolderId(WellKnownFolderName.Calendar,
            "fx@msdnofficedev.onmicrosoft.com"),
            filter,
            view);

        foreach (Item item in results.Items)
        {
            Console.WriteLine("Subject: {0}", item.Subject);
            Console.WriteLine("Id: {0}", item.Id.ToString());
        }
    }
    catch (Exception ex)
    {
        Console.WriteLine("Exception while enumerating results: { 0}", ex.Message);
    }
}

private static void GetDeligateCalendar(string mailAddress, string password)
{
    ExchangeService service = new ExchangeService();

    service.Credentials = new WebCredentials(mailAddress, password);

    service.TraceEnabled = true;
    service.TraceFlags = TraceFlags.All;

    service.AutodiscoverUrl(mailAddress, RedirectionUrlValidationCallback);

    SearchFilter sf = new SearchFilter.SearchFilterCollection(LogicalOperator.And, new SearchFilter.IsEqualTo(AppointmentSchema.Subject, "Discuss the Calendar REST API"));
    DelegateAccessSearchWithFilter(service, sf);
}

And if you want the Outlook and Graph API to support this feature, you can try to contact the Office developer team from link below:

https://officespdev.uservoice.com/