That ‘free’ stamp is not guaranteed. They are at liberty to refuse entry. The 60 days is intended for tourism. If you are staying long term there are other visa options.
I have helped girlfriend(s) to establish a small business. From my experience, most aren’t business savvy. Or, perhaps they are! If they can get the falang to buy the stock then they make a good profit when they sell it. Of course, farang now has to replace the stock!
Make a business plan. Identify all set up costs and all running costs. Do projections of expected revenue (worst case and best case). Assess the financial viability of the business and make sure your partner is fully aware.
When you give her the hard facts that the business is not viable, I would love to see her reaction!