1) Yes she can file such a complaint without proof. Has your wife sent this complaint to other people (relatives, employers, newspapers etc.)? If not - there is no defamation.
2) Your friend cannot be charged under 498A. You and your family can be charged. Your friend's name is added only for purpose of masala.
3) All 498A allegations are a copy-paste sob-story. Bechari abhagi abla naari. She has to prove her allegations in court. It is impossible to prove false story.
4) Nobody will bother you or your friend while this drama is unfolding. People may try to scare you about arrests. These days arrests don't happen in 498A cases - infact very little actually happens.
5) Find experienced lawyer, follow proper process and navigate the situation. Some money will be spent on legal fees. This is a tax for marrying stupid woman.
6) You wife has bought herself a bitter pill from some lawyer and she will realize this in due course.
7) Have patience. Sit back. Buy some pop-corn.
8) Filing counter cases is a waste of energy.