I attempted to do this to my ex while we were seperated and was told that I was allowed to do it by my HR director and gave me the paperwork to do so.
I then asked my divorce lawyer told me that I was free to do this that since Ohio is a community property state (same as Wisconsin) my soon to be ex could stick me with half of the bills as we are still legally married. He also told me that if my soon to be ex went out and got private insurance (at the tune of about 1200.00 per month), that I could get stuck with half of it. I soon rethought my maliciousness, as it was just solely designed to piss him off. (being honest here). I was not willing to put myself at risk, but I'm assuming that since my lawyer said its true, I listened.
With that being said...it was very unwise for him to do this. Your friend needs to get an attorney either way. He is still married and may be reponsible for half of any medical bills that she incurs while they are still legally married espically with no divorce papers in the process. Since it was just a malicious thing to do make sure your friend get a shark for a lawyer.

