Originally Answered: How to send an invitation letter for a visa?
If a foreigner applies for a visa to the US, the US consular is trained to assume that they want to remain in the US illegally, as stupid as that may sound. In order to get a B2 approved, the petitioner has to show that she has strong ties to Japan, i.e, a top job, property, money in the bank, family, etc, and no ties to the US. If you send her a letter of invitation and she shows it at the consulate, they will deny her application because she has a support system in the US where she could hide and get fed while trying to escape deportation.
Sounds crazy? Yep, but this is how the cookie crumbles.
For that very reason parents can't even visit their children and get to know their grandchildren, because the consular officer has established ties to the US.