If you use a copyrighted work in a way that is normally an exclusive right of the copyright holder (redistributing the work, creating a derivative work, etc.), and your use has not been licensed by the copyright holder, then you are committing copyright infringement. The decision of whether or not to sue you for copyright infringement is entirely up to the individual copyright holder.
That said, if you've downloaded the open-source version of the code, you have already been granted a license to use the work under the terms of the GPLv3. If you don't want to pay for a commercial license, then you must follow the terms of the GPLv3. If your use correctly obeys the GPLv3, then your use is licensed and you cannot be successfully sued for such use. (Note that they could still file suit against you -- anyone may do so at any time, for more or less any reason -- but their suit will almost certainly not prevail, because it would be completely baseless.)
If you use violates the GPL, then you would be operating outside the scope of the license grant and you would be liable for infringement. For example, if you do not also make your work available under the GPL (or a GPL-compatible license) when you distribute it, and appropriately make your human-readable source code available, then distribution of your code combined with Isotope would violate the GPL.
If you are selling the code you write to your clients, and they do not wish to abide by the terms of the GPL, I would suggest you (or your client(s)) purchase a license to relieve them of the requirement to make their source code available.