She wants to roll over the security deposit for the additional six months.

To keep the tax-sheltered status of an IRA or 401k, all investments must be done at arm's ength.There is a program known as a Rollover as Business Startus (ROBS Plan) that would allow you to use existing IRA or 401k savings to capitalize your own business.  

@Shaun Skeffington is referring to called 60-days rollover, not a loan.

They should be able to roll over their traditional employment 401k into a RIRA or IRA.

But now as I do exchanges, that exception doesn’t roll over to those new properties. 

Solo 401k will accept rollovers from any other retirement plan except Roth IRA. 

Please see additional considerations below.If you are eligible to set up a self-employed Solo 401k (or have a 401k plan through an employer which accepts rollover contributions and allows for 401k participant loans), another alternative which would avoid taxes and penalties would be to transfer your funds to such a 401k plan and then take a 401k participant loan.

@Thomas RutkowskiJust for fun, let's use your numbers:20% ROI8% growth rate4% loan28% tax rateWith my standard investment account, I contribute an additional $100,000 each year and rollover the profits.

You may avoid the taxes if you deposit the funds in an eligible retirement plan (which includes anIRA) within "3 years and a day" of the date of the COVID-19 distribution (note: compare to a 60-day rollover).

To learn about the new one per every 12 month IRA rollover rule visit:Internal Revenue Service (IRS) Announcement 2014-15