Can the COBRA premiums for an employee's spouse be reimbursed through the 125 plan?

No, I have always heard that premiums are not eligible expenses for reimbursement. Unless there is some weird way around it that I don't know about.

Premiums cannot be reimbursed from a Medical Reimbursement account. A cafeteria plan may allow for reimbursement of individually owned insurance (IOI) policies, but it is a separate account.

Even if they had the option in their plan to be reimbursed for IOI , reimbursing a spouse's COBRA premiums is a bad idea as it may be impermissable participation in the plan by the spouse or dependent. The only insurance coverage that can be run through the plan is coverage belonging to the employee or coverage under the health plan of the employer sponsoring the plan.

This comes from the EBIA Cafeteria Plan manual.