Telkom Marconi is easy to hack (because it is hard to configure properly - but don't tell him that!) and that your risk of someone stealing your login info, and capping your account is very high.
However... if you get the dlink... bla bla bla.
That should do it
Tell him that with the Telkom, "people can steal our internets".
Seriously, though, the Telkom router only supports WEP which is easily hacked, whereas the D-Link should (hopefully...) support WPA, which has not yet been cracked.
Tell him I work in IT and install these and other routers and unless he wants an unstable connection which is slower(sad but true ... donno why), a wireless connection which keeps stuffing out... unless you reboot the router about min once a week.... HE must get a better one.
I suggest D Link or netgear as both of these have never given any problem.