Terminated without cause immediately after salary renegotiation equity agreement

Employment is "employment at will" in this country. This means that except as set forth below, you may be terminated at any time, for any reason, without notice and without recourse (i.e. you cannot do anything about it--"tough luck," as you put it). This includes termination during or just after a salary or equity negotiation. So the law as a general matter is, this is legal and your wife is not entitled to keep her job or to compensation.
That said, 1) if there was a written employment contract which was still in effect (i.e. unexpired) and which prevented termination at this time or for this reason, your wife could sue for "breach of contract"; and 2) termination for one of several specifically prohibited discriminatory reasons is illegal and your wife would have a claim. The principal forms of illegal discrimination are based on race, color, national origin, age 40 or over, disability, religion, or sex: termination cannot be based on those reasons. That does not mean that a racial or religious minority, a woman, a person from another country, etc. cannot be terminated, of course; but it does mean that their membership in that category cannot be the reason for termination. There must be some other reason: performance, cost savings/restructuring, violating company policies or insubordination, etc.--something justifying termination other than race, sex, age over 40, etc. If your wife believes there is no reason for her termintion other than being, for example, a woman or over 40, then she should contact the federal EEOC or your state's equal/civil rights agency about filing a complaint.