Unless she has urgent need for the extra cash now, retaining the tax relief later is usually recommended.
The amount she can put towards an AVC in her final payslip should be coming out of taxable income (and not the tax free part of the redundancy or the statutory tax free bit) to maximise relief at 40%. Something like [ex gratia]+[PILON]-[SCSB]=taxable * 25% (age related limit).
She should definitely make any AVC before leaving employment, as if she tries to do it after leaving this job and before starting another, Revenue may deem any contribution to be out of the tax free lump sum, with no tax relief allowed.