To solve issue without installing AJAX.NET:
Copy AjaxControlToolkit.dll and AjaxControlToolkit.pdb version 1.0.61025.0 from my system (in which asp.net ajax is installed) to ASP.NET App bin folder of my friend's system.
(i install my AjaxControlToolkit in C:\Program Files\Microsoft ASP.NET\ASP.NET 2.0 AJAX Extensions\AjaxControlToolkit)
Copy System.Web.Extensions.dll (C:\WINDOWS\assembly\GAC_MSIL\System.Web.Extensions) and System.Web.Extensions.Design.dll (C:\WINDOWS\assembly\GAC_MSIL\System.Web.Extensions.Design) from my system to ASP.NET App bin folder of my friend's system.
Run the website and it's working fine.