It’s been a while since I took corporations in law school, but I’m pretty sure both the waitress and the lawyer in your case have broken the law. Insider trading based on information about mergers has different rules and doesn’t require a fiduciary relationship. (Quick googling suggests the case that matters here is US v O’Hagan)
That being said, yes, in a more vanilla insider trading case, there certainly is a moral distinction between taking advantage of information that someone entrusts you with with the expectation that you’ll keep it to yourself and taking advantage of information you find out by being a waitress in the right place at the right time.