This sounds odd to me because a solo 401k plan, by its nature and legal structure, does not (usually?) have the safe harbor feature that is found in traditional 401k plans because those rules ensure that the plan is not discriminating between the employees and HCEs or owners. Solo 401k plans are specifically structured for owner-only businesses without full-time employees (except the owner(s) and possibly their spouse(s)). If the owner(s) hire full time employees, they may want to implement a plan with safe-harbor features so they could contribute the safe harbor contributions without testing but that wouldn't let them provide an additional 21% nonelective contribution just for the owner(s) as any contributions in excess of the safe harbor would have to be tested under 401(a)(4).
That said, back to your actual question, annual additions paid to a participant's account cannot exceed the lesser of: 100% of the participant's compensation, or $70,000 (assuming <age50) for 2025. But, as you state, there is also the deduction limit where an employer’s deduction for contributions to a defined contribution plan cannot be more than 25% of the compensation paid (or accrued) during the year to eligible employees participating in the plan. Since the 25% limit is on the employer's deduction for contributions to the plan it does not apply to the salary deferrals as the deduction for those deferrals is really for wages paid. See 404(n).
So with $100K compensation, assuming these are W-2 wages, she can contribute $23,500 (assuming <50) and she as employer can contribute an aggregate employer contribution of $25K in 2025. Note that I am assuming these are W-2 wages. If her compensation is actually earned income, there will be a difference (see Self-employed individuals: Calculating your own retirement plan contribution and deduction | Internal Revenue Service). Also, this example assumes only one owner and no spousal compensation.