If the friend’s router is also a Linksys and similar model (e.g., both in the WRT54G family), you *might* be able to use WDS (traditional wireless bridging). However, because WDS is not a wifi certified protocol, implementations vary and compatibility is always an issue, esp. across manufacturers (and sometimes even within the SAME manufacturer). And since it requires adding the WLAN MAC address of each router to the other’s WDS configuration, this really has to be a “friend”, not just any ol’ open wifi hotspot you may be “borrowing” from a neighbor or other source for Internet access.
If your router supports dd-wrt or tomato third-party firmware, you could install either one and then configure it as a wireless Ethernet bridge.
Of course, that assumes you're not using the Linksys as YOUR primary router. This works w/ ANY wireless router because it connects like any other USB/PCI wireless network adapter you might find on a desktop/laptop PC. IOW, it has no WDS dependencies.