yes you can
CODE SECTION 401(c)
(1)Self-employed individual treated as employee
(A)In general
The term “employee” includes, for any taxable year, an individual who is a self-employed individual for such taxable year.
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a 401(K) plan can receive two different kinds of contributions
a) elective deferrals by an employee (your spouse per the above definition)
b) employer contribution
there is an overall limit for the year provided the plan does not have limits lower than allowed by law
for 2022
a) the employee(your spouse) can make an elective deferral of up to $20,500. if 50 or over this rises to $27,000
b) the employer (the sole proprietorship) can contribute up to 20% of net SE income ie schedule C income less 1/2 the SE tax
c) the max of both for 2022 is $61,000 if under 50. $67,600 if she's 50 or over. IRC code sec 415
