Assuming this in on a Mac, she can simply delete his login info when it auto-fills, and put in her own information. She’ll be asked if she wants to save that information to keychain, and she should say “yes.” The next time she arrives on that page, the Mac will offer a choice of which login to use.
If she’d like to remove his login information altogether, just open Keychain Assistant, look up the website, find the entry with his information on it, and delete that entry, leaving only the new one. Please make SURE that he has his login information for that site available and working on his computer before deleting his keychain entry.