Can you sue your lover's wife for harassment?

If she has been harassed, yes.

The legal definition of harassment although the laws differ from state to state, is when a person(s) have been intimidated in some manner such as being threatened or placed in a situation of potential danger not of their own making.

The burden of proof in all lawsuits rest with the plaintiff, this is especially true when it relates to harassment, discrimination, stalking and so forth. Any person wishing to pursue such action should be very certain of the validity of the suit or he or she could end up with legal problems also.

It might be a good idea to make oneself aware of the laws of their state pertaining to such acts or obtain the advice of a qualified legal professional to be assured they have a chance of prevailing. Better to do a little research then to spend money (filing fees, court costs and perhaps incur a counter judgment claim) on a unviable suit.