What Brigid says has been my experience from a Benefits Manager's perspective. However, I have worked for an employer who allowed ex-spouses to remain on the medical plans until age 65 or until the employee or ex-spouse remarried. This liberal a policy, I might add, is not very prevalent among employer health plans.
If this is the policy of you or your friend's company it should be described in their Summary Plan Description (SPD). If you or your friend are currently covered as a spouse, ask the company benfits person for a copy of the SPD.
Unlessd you see it in writing, I wouldn't rely on word-of-mouth from the employer or the insurer.