The deadline to open the solo 401(k) is the due date of the tax return, including extensions, but for 2022 only an employer contribution is permitted if the plan is established after the end of 2022. The SECURE 2.0 Act changed the tax code to also allow elective deferrals to plans established after year-end, but only for plan years beginning after 2022, which is not the case here. What this means is that the amount that can be contributed to a solo 401(k) for 2022 is the same as the amount that can be contributed to a SEP IRA for 2022. In your case the deadline for your wife to establish either plan and make the employer contribution is October 16, 2023.
Given that no elective deferrals (or Roth contributions) to the solo 401(k) will be possible for 2022, there is no concern about your wife exceeding the elective deferral limit for 2022 across multiple plans. With a solo 401(k) established to accept 2023 elective deferrals, the per-individual elective deferral limit will become relevant.
