Is H4 from Prospective employer valid even if I (primary H1B holder) don't join them?

I am on H1B from employer X. My wife is on B2 in USA. Prospective employer Y applied H1B transfer for me and B2-H4 conversion for my wife. Now both are Approved and we got receipt. For my wife, as part of Approval Notice, she also got I94 with new validity period of 1 year (it was 6 months on B2). But I have decided not to join Y. Does my wife go back to B2 or is her H4 still valid? Can she legally continue to stay beyond the 6 months granted in B2?