"port her cellphone number to a VOIP provider that does support receiving SMS from shortcodes over wifi"

...

"... unless the company she's dealing with is one of those that don't send SMS-OTP codes to VoIP numbers for seCuRiTy reasons ..."

Correct.

This is, in fact, a terrible idea because even if you do find a VOIP provider that can receive SMS from "short codes" (the weird little numbers your bank sends codes from) that is a temporary oversight and will get "fixed" eventually.

Remember:

None of this is for your security or to help you. All of these measures are just sand in the gears to slow down the relentless onslaught of scam/spam traffic.

Your bona fide mobile phone number is a "proof of work" that these providers are relying on in absence of any real solution to this problem.