Often times there are software conflicts that can happen between computers and Yahoo that can be resolved.
Which virus scanner do you use? Does it filter web pages?
How high are your security settings? Are you blocking cookies from Yahoo?
Try some of the alternate login pages at the link above.
Have you tried using a different web browser? IE vs Firefox vs Chrome ?
It might be easier for you to list everything you have tried so I can come up with some more suggestions for you.
Their support won't add it to their answer, but yes, sometimes there are goofy problems that happen with Yahoo accounts that take some time to resolve themselves such as hidden emails (wrong inbox count) etc.
Can both of you list what software is on your computer?
Hopefully we'll be able to find a common software that can be fixed.
Is all of your software up to date? A
Secunia PSI scan sometimes helps.