Tell the engineer before work starts, even if the cleaner seems to have drained away. Give the product name, approximate amount and when it was used, and keep its packaging or a clear label photograph.
What the observation tells you
A blocked connection can retain chemical liquid out of sight. Opening the water path without knowing what it contains may expose the person working to splashes. Do not add another cleaner, bleach or a home remedy to neutralise it.
A useful next step
Leave the disposer off and follow the product's exposure and emergency instructions where relevant. Describe any other chemicals used in the same sink so the clean-up can be planned safely.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.