"and as a result the deduction."

If they are avoiding the employee relationship for legitimate reasons, then look into Spousal IRA provisions.

If they qualify, you could have a Qualified Joint Venture, which means you would split it not just for retirement plan purposes, but consider this is where a spouse is often shorted on Social Security credits, bu not "working and earning" in the business.

Here's a in depth reference:

https://www.mysolo401k.net/solo-401k/