I don't see an issue with them taking the enhanced startup credits and tax credits for 2022, given their participation in the PEP, since 2.0 clarified participation in a PEP was treated the same as a single employer establishing a plan.
To answer your question, the ER's participation in the PEP was in fact identified by name, EIN, plan number, etc. and their identifying information is the same in the newly established single employer plan, so I agree, it's "the same plan". Along these lines, it seems reasonable they would remain eligible to take the enhanced startup and tax credits for 2023+. The fact that their payroll company recommended they become an adopting employer in a PEP, which turned out to be a really bad idea, and now they have to incur additional costs to set up a new individual 401(k) plan to make things right, may be irrelevant.
But I agree with you, I will recommend they defer to their CPA for final interpretation on this.
Thanks for your input and any additional input on this.