None. Considering his stay was indeed to be 3 weeks, if he lied (saying that he was entering as a tourist), he would get in and be given a 3-6 month stay period.
And after you are in, US customs do not have any way to see what you've actually done.
It's going to be hard to get a good answer on that because anyone who knows significant numbers of foreign people who work as "consultants" in the US on the sly and could provide a data point aren't silly enough to spill the beans (hopefully!)
Whats the worst that will happen? They're already sending him home.
It could be worse. If he is caught lying (not that it is easy, see my comment above), he can be denied entrance to the US for 5-10 years.
Being denied entry is enough to be excluded from the Visa Weaver program. He needs to get at a minimum a 6 month multiple entry tourist visa in order to enter the U.S. now. They will still take him for extra questioning each time he enters. It will take hours each time.
How does the US correlate the two files if you change your name in your home country?
When you apply for a visa, they take your fingerprints.