score:2
The answer to your question is indeterminate and there is lots of variation in the 'official' answers you may receive. It means people in credible, authoritative positions do not totally agree.
I attended a law course in Schengen designed for practitioners last year and this question was raised specifically. There was wide-spread agreement from the instructors that said the 'best practices' strategy was to exit Schengen and to immediately reenter as a short-stay visitor. Doing this provides the opportunity to show absolutely that you are not overstaying your residence permit. It also shows that you have had a landing interview and are officially cleared as a short-stay visitor.
The other strategy is to let the residence permit expire and to rely upon its 'implicit' conversion to the standard 90/180 rules. So as long as you are within 90 days of your residence permit's expiry, you are OK. This strategy is more convenient but as mentioned relies upon an 'implicit' conversion.
It is doubtful that either strategy will prevail over the other in the near term. Your question is...
She said “no” and that I will have to leave the EU and go back to US for three months. This is where it didn’t make any sense. I know that the UK isn’t a Schengen zone and my expired German resident permit has nothing to do with it. Am I right?
You are correct. All sides would agree that the US is an incidental factor and not a required travel destination. The three months requirement is not relevant either, but you would have to ask your lawyer what her references and authorities are to make sure. Or speak with someone else. Bear in mind that this answer does not slag off your lawyer, it means there is no apparent authority governing her advice.
Of the two strategies outlined above, the first is preferable because you would be lawfully documented as a short-stay visitor and free from awkward debates about the 'implicit' conversion of your stay in the Schengen zone.