<human>: Can you give me the gist of the text in a nutshell?

While my wife was on maternity leave from the start of November '14 through mid January of this year her employer somehow changed the allowances on her W4. 

I found this looking at a bunch of pay stubs she brought home from work yesterday. She thought the increase in her check was due to the promotion she received in February.  She only ever checked her PTO balance and pay on her stub.

She originally was at 2 allowances on federal but now its 7 for federal, 5 for state and 3 for local (NYC). So basically she's been paying next to no taxes for 3 months. I went over her leave forms and work paperwork that she needed to do and she never submitted a new W4 while on maternity leave.

How could the employer do this? What's to do next to avoid owing significant money when doing taxes next year? This really pisses me off and would be raising hell with my HR dept. if it was me but she is far too nice. I would love to pursue how this was changed for her and hopefully find out how or who did this.
<bot>: Wife's employer drastically increased allowances on W4 without consent, paid virtually no taxes for 3 months. How do I reduce the impact for next year's tax time?