Risks of valuing a business too low

I'm working with a client that wants to gift 20% of their business to a key employee and wants to keep things "simple" for themself and keep their legal bills low (they are aware of the tax affect his will have on their employee). Because they won't have to file a gift tax if the value is under $30,000 (yes, they're married) the owner wants to say the business if worth $150,000. While I am by no means a valuation expert, I would guess the business is in the ballpark of $600K - $800K; much higher than $150K. My standard advice in this situation is to have a professional valuation done to substantiate the value should they ever get audited. However I know they have no interest in paying several thousand dollars for that, especially as it will likely result in them needing to file a gift tax return, pay higher legal fees, start dipping into their lifetime exemption, etc.

I realize that if the client wants to do this, they will, and ultimately its on them. However, I also want to ensure I'm doing my due diligence in providing them all of the relevant facts.

If the gift is under $30K and no gift tax return is required to be filed, I'm thinking there's a very low risk they'd get caught by the IRS valuing their business too low. If anyone has been in this situation before, I'd be curious to hear if there are any risks I'm missing?