I would say that the lawyer is not quite correct, for the following reason:
Once you file I-485, you can only continue to work on TN as along as (a) it doesn't expire AND (b) you don't leave the country. that is why it is recommended to file I-485 when you have sufficient time left on TN and are willing to stay in US for a period (3-4 month minimum)
As curiousgeorge warns, If your I-140 is rejected/delayed/RFE'd, it may take time to either fix the I-140 or -- if I-140 cannot be fixed -- retract the I-485, which may be too late for your to continue on TN (ie. if it expires of you need to leave US for some reason).
It is unclear how long a "dead" I-485 stays on your file, but, I would think that that I-485 would be clear immigrant intent for quite a while.
So, if you file concurrently, and I-140 is rejected, yes, you can still work on your current TN, but only until it expires, or until you leave the country, whichever is soonest. That is not the best situation, and can be avoided by waiting for I-140 to be approved before filing I-485.
is there some particular reason that your lawyer is unwilling to wait to file I-485?