Most medical premiums deducted from your pay are deducted on a pre tax basis. That is they are already deducted from your taxable pay. As such, you cannot deduct them again. So, not only is the portion of medical insurance that is for her not deductible, the entire premium is not deductible. If you are not sure if yours are pre-tax, you need to ask your employer.
If the premium is after tax, you may continue to deduct your spouse's portion, as a medical expense, as long as you are legally married. After the divorce, you cannot. You have a whole other issue of whether your employer will continue providing insurance after the divorce.
If your question is can you deduct the pre-tax premium as alimony (spousal support), the answer is no. Pre-tax means it's already deducted from taxable wages and you cannot double dip. If your question is can you deduct an after-tax premium as alimony, the answer depends on the terms on the separation agreement. If you deduct it, be sure she knows she has to report it as alimony income
