There was an earlier post with the same question. I copied the best answer and the link so you can read the other posts if you want.
“Unless you are rooted, I don't know of any way to do it. If you are rooted, or are willing to root your Galaxy for those nice guy points, you should be able to use a file manager (ASTRO, Root Browser, etc.) to find it.
Use the file manager to locate your data/misc/file folder, then look for wpa_supplicant.conf, or I assume it could be wep_supplicant.conf if her network is using WEP instead of WPA. Open the .conf file using a text editor (which is probably built into your file manager application, if not, add that to your shopping list). You should be able to read the password in plain text at that point, and start enjoying your new minor league hero status.”