I did the research for the requirements to use Qt LGPL before (NOTE: idk if all these things are required, maybe the Qt Company is exaggerating when they claim all these things are required, someone more familiar with LGPL please chime in).

1. Product manual must mention you use Qt LGPL, and include the text of LGPL

2. Graphical software (desktop or terminal) must mention you use Qt LGPL, and include the text of LGPL. Usually this goes in Help -> About/Acknowledgements.

3. If the customer requests the source code, you must provide it on request. They can contact you and you mail them a USB flash drive or download link. Most companies seem to pre-emptively place the source on their website and include a link to it.

4. You must allow the user to rebuild/replace the Qt libraries your program uses, including instructions on how to do this if it's not trivial.

So in your case, simply email the developer and request the LGPL source code and instructions on how to build/replace the product's LGPL libraries.