you could attempt to see if he has messenger service enabled and utilize that

NET SEND
{name | * | /DOMAIN[:name] | /USERS} message
so you would connect to his wide open linksys wireless network and do a quick IP scan to see where his computer is located (ie. 192.168.1.2)

then do a net send 192.168.1.2 hey this is xierox, i wanted to give you a heads up that your wireless network is wide open to anyone around in the area, this is a huge risk send me an email at [email protected] and I will either walk you through it or for 80 bucks ill lock it down and teach you how to use it.

edit: and if he doesnt have messenger service enabled then just rename his ssid to UNPROTECTED or EMAIL tempemail@hotmail for help heck with it wide open his printer is prolly shared too so you could connect to his printer and print out your own flyer that says to call you for help