Having lived in the GPL/LGPL world since 1994 (BRU was the first commercially available application for Linux and FreeBSD/BSD386), what we are doing here doesn’t require legal advice as we are just discussing the general nature of these licenses. My lawyers have made it very clear to me and my team as to where the line is drawn. However, if the discussion leaves you with the slightest doubt, communicating with an Intellectual Property lawyer is definitely the way to go.
First, if you are using an LGPL library and the creator is not providing the source code, they are already breaking the rules of LGPL. You should ask them about the source code and remind them that it’s required for their tool to be LGPL.
Here’s an analogy that a member of the FSF team came up with back in the early days of the GPL as to why the LGPL was needed:
Imagine if concrete was a licensed building material and that every time you built a building that used concrete you have to turn ownership of your house over to the manufacturer of the concrete. That analogy is the GPL - the concrete (library/APL) is restricting your ownership of your building (App) - you can’t build the building without the concrete, but the restriction for use of concrete forces you to give up your work.
In another instance, the concrete manager retains ownership of the concrete, but allows you to build your building and retain ownership - so long as you don’t claim ownership of the concrete used and don’t restrict the person who buys the building from you from replacing your concrete with another brand or type of concrete. That is the LGPL. You still need the concrete, but the manufacturer of the concrete let’s you retain ownership of your building design and construction.
In a third scenario, the manufacturer of the concrete has released the formulation to the world and allows you to use it, modify it, claim it for you own, and take full responsibility for it. That is the BSD license.
In almost any compiled app, you depend on a lot of external libraries for the app to run - especially on Mac OS where they do not allow you to statically compile your app (that is a very different rant of mine). if you dig deeply into the license text for most development tool sets, you will find text that states that you are allowed to link against the development platform libraries without assigning rights to your application to the creator of the libraries. Imagine if every application running on Windows or Mac OS had to be handed over to Microsoft or Apple just because you were using their graphics API and libraries. The Xojo Framework libraries that are included in your compiled project is another example - since they provide a license for you to build commercial applications that use the functionality of the Xojo frameworks, you don’t have to assign the rights and ownership of your application over to Xojo.
The LGPL simply means that as long as the .so/.dylib/.DLL library file that you are using in your application is separate from your compiled code (supposedly so that the user could replace the said library with a different version), you can call the functionality of the library from your app without the need to release your app as open source. The only restriction being that you make the source code available if a user requires it and that if you make any modifications to the LGPL code, you submit those changes back to the original author(s) of the LGPL code.