You can either
- Follow your lawyer's advice and apply for an E-1 visa, or
- Try to find another lawyer who will give you different advice.
Proceeding with the VWP on the basis of advice from internet strangers is ill advised.
Whether your engineer can qualify as a business visitor depends on a subtle determination of the nature of his work in the US. There is a lot of gray area between things that are clearly allowed on a B-1 visa (or the VWP) and things that are not. A good immigration lawyer will be familiar with relevant case law, especially cases heard in the immigration courts, and be able to evaluate the specifics of your case in light of these precedents.
If your engineer continues using the VWP and is refused entry at some point, it will be seriously problematic for him. He will probably never be able to use the VWP again, and having tried to do so after being told not to could well make it difficult for him to get a B (visitor) visa.
Refusal of entry under the VWP cannot be reviewed or appealed; the only recourse is to apply for a B visa. If the embassy refuses to grant a B visa, or the CBP immigration inspector refuses to admit him in B status, you'll be spending even more in legal fees, and he will have a negative immigration record to worry about. So a good immigration lawyer will probably advise borderline cases to go for the E visa, because you would probably rather do that than litigate a test case in the immigration courts.
Still, without knowing the facts of your case, nobody can tell you how likely it is that you'll find a lawyer who thinks that your engineer clearly qualifies as a business visitor. If you can, the lawyer should be able to help explain the best way for your engineer to present himself at the border to make his eligibility for that status clear to the immigration officer.