A quick Google seems to suggest that rather than being the AVC provider, Wise run a salary sacrifice scheme for LGPS employers, with the funds then going to the employer's usual AVC provider.
She needs to check the details, but it looks like a no brained to go this route. A normal SIPP would just save the tax. It looks like the Wise route also saves her NI and possibly some of the employer NI savings. I can't see any benefit from having a better fund choice in a SIPP being enough to offset that - and she could always transfer the funds out later.
On top of all that, she might be able to leverage her DB pension to take the whole of the AVC fund tax free at retirement.
If thus had been available when Mrs B was an LGPS member we would have bitten their hands off.