pinging default gateways

A user was on her computer, the internet worked fine, she went to ipconfig and gave me her default gateway and ip address. From a remote location, right then, I could not ping them. Why would this happen?
(I am looking for a certain method of monitoring uptime at various offices. I was gathering the default gateways and using a site-monitoring tool to auto-ping the default gateways. But this has me worried my method is not so good.)