The only possible way to keep your ex on your employer’s group health insurance plan is to file for a legal separation instead of a divorce (see my "Legal Separation or Divorce" post).
Otherwise, regardless of what your ex expects or demands, the good news for you is: YOU CAN’T. Not through your employer’s group plan, anyway. And not even if you wanted to! You can only insure family members through your employer’s group plan, and your ex is no longer a member of your family!
However, if you’ve been covering her up to your divorce date, and your employer has more than 20 employees, you might want to let her know (if you want to) that she’s eligible under COBRA for coverage equal to yours for 36 months after your divorce. She’ll have to pay for it out of her own pocket, of course…but at least she’ll be able to get health insurance if she can’t get it through her own employer.
And don’t go and try to be “Mr. Nice Guy” by keeping her on your health insurance even after your divorce is final, because it could bite you in the butt! A friend of ours did, and ended up having to reimburse his employer for every last penny that they’d spent toward her health insurance from the date his divorce was final! We were lucky: my husband waited a couple of months before dropping his ex, but because their end date was fuzzy (it was left open because it was based on her being seen by a specialist first), his employer let it slide. Same employer as his friend, but different circumstances...so, again, be careful!
My husband’s divorce took YEARS, and in all that time the devil was eligible for FREE health insurance through her employer. But she refused it out of spite…she wanted my husband to have to pay for her coverage because it was costing him an additional $5,000 a year! Well, as you can imagine, my husband took great satisfaction in finally being able to drop that bitch from his plan! But to add insult to injury, when he finally DID drop her--2 years after he should’ve been able to--she actuallly had the nerve to be petulant and insulting about it! What a surprise!