Is it illegal to use somebody else's address?

Those things are mailed out based on purchases of "headers" from credit reporting agencies. The credit reporting agency would have records tying her to her ex, change of address for the ex, etc. Then these things get sold and resold. It happens. Over 70% of all info held by credit reporting agencies is erroneous.

She would not have submitted any credit application, nor would she have given out her ex-husband's address. She has done nothing wrong. In fact, she has done nothing whatsoever. And she knows nothing about this at all.

Stop obsessing and toss the blasted thing in the trash. It's junk mail!

Yes, it's considered forgery. Or in other words, fraud. Call the company and tell them that that person no longer lives/works here. Unless, its another kind of situation that may involve the husband.

It's quite a common mistake. Just toss it and fuggedaboudit.