score:4
The advice on this site is consistently “never, ever, lie” on any visa application or to a border official, and rightly so.
Will the US automatically know about that travel? Probably not. Can they? Yes. Depending on the circumstances it can range from extremely easy (operations on a bank or credit card statement, photos on your social media accounts…) to quite difficult, but why take the risk?
Is the risk low? Probably. But the consequences can be quite severe. If they want to take the risk, they need to be aware of those consequences. Applying for a visa is annoying. Being banned for deception can be a much bigger problem, depending on their circumstances.
Not having the stamps makes it (in most cases) less obvious. It does not mean there’s no trace of the visit.