Of course the biggest irony is that in trying to save her he condemns others, but the fact that she is in there at all is deeply ironic, not only because she is an upstanding citizen but also because of the role the "problem" of her reading plays in her condemnation. Her reading mystifies her husband and he feels threatened by what he doesn't understand. Although there is an absence of malice in his questioning of her activity, his response to her reading nicely dovetails with the town's response to the imagined threat of witchcraft: fear and insecurity in some leads to the destruction of others.
Francis Nurse had made mention of "books" that his wife was reading, and at one time muttered about not understanding what she was doing. This becomes misinterpreted in the hysteria of accusations,so that her husbands own words are used against her.
Trying to help his wife, along with Giles Corey and Proctor, Francis Nurse brought a "testament" with ninety-one signatures on it of people who are farmers and members of the church. The people who signed it attest to the fact that they've known Rebecca and Martha Corey for many years, and they've never seen them have anything to do with the devil. They are swearing that both Rebecca and Martha are good women. Because their friends and neighbors were willing to sign this testament, they will now be arrested and brought in for questioning. So in his attempt to help his wife, Francis Nurse has now brought the wrath of the court down on ninety-one more people.
The first poster is correct. The second and third ones are talking about Giles Corey. Corey also tries to help his wife and is actually a much more interesting character than your question's focus.