COBRA isn't a plan, it's a rule that says your employer (or employer group or union etc) based health coverage needs to allow you to continue participating in the plan after you lose eligibility. When you find yourself in a COBRA enrollment window, you are only permitted by the rules to extend the elections you had already made. As an example, if you were covered by the HDHP as employee only, you can't change that to the PPO as employee + spouse until the plan's open enrollment.
For COVID ARPA included some emergency COBRA relief provisions required in 2021, but to my knowledge those have all since expired.
COBRA is not her only option, her coverage loss would give her an enrollment window to purchase coverage from either your state or the federal exchange. But your employer's plan is generally not an option as your COBRA administrator has indicated. It's early enough in the year that accrued deductibles or plan maximums probably aren't an issue. At this point, with guaranteed issue individual coverage available, the most common reason to elect COBRA is the person has already met the plan maximum and has scheduled treatment.
