Employee has coverage under spouse plan. Spouse plan has open enrollment effective January but employee's employer has it 7/1.

Employee wants to drop coverage under her spouse plan effective January 1 and come on to her employer's plan.

Is this allowed?

HIPAA only requires that a plan allow an employee on outside of open enrollment if they have exhausted their COBRA rights, or, if the coverage is not COBRA, if the individual becomes ineligible for coverage or the spouse's employer ceases contributing to the plan. The employee dropping coverage voluntarily at the spouse's open enrollment is not a HIPAA event. HIPAA gives the bare minimum. The plan may be written such that a drop at open enrollment is a loss of coverage, but the bare HIPAA rules won't help.

As an aside, keep in mind that the 125 rules (if this is a 125 plan, as well) allowing election changes when one spouse's open enrollment period is different than the other spouse's will not help. They only say that pre-tax 125 elections may change under these circumstances. They do not override the provisions of the underlying health plan.