Yes, I reviewed the applicable law. It is possible that the ex will be able to raise a set of facts that would make the new partner's income relevant. However, relevance is one of the best arguments to make to get a subpoena quashed. It may also be possible to argue that the request is overly broad and unduly burdensome, depending on how much information is being requested. For example, a request for 10 years of bank statements is probably going to be overly broad and unduly burdensome.
I didn't address this earlier, because you were only asking about quashing the subpoena, and I was addressing ways that could be attempted. However, if the judge finds that she's in a relationship similar to marriage, such as cohabitiating with another person, there's a good chance that he'll terminate her alimony. See Taake v. Taake
, 70 Wis.2d 115 (1975). However, this has nothing to do with the new partner's income. It only has to do with the fact of the cohabitation and the idea that a former spouse shouldn't have to pay for the ex to live with someone else. Thus, if they're living together, sexually cohabitating, and sharing expenses, his income shouldn't matter - her alimony will terminate no matter what his income.
If the Motion is quashed regarding the financial information, it's possible that he would still have to appear to answer questions regarding their relationship, unless the parties can agree to stipulate that she is living with a new partner and that his appearance is not necessary to confirm that fact.