Is he correct? That you can't have GPL files in your project without all code adhering to it? I thought it has to be linked static. So just calling a GPLed js library likely wouldn't be enough. I think the law is muddy here and not clear at all, even if the code is directly bundled.
First, if you are distributing modified code or code compiled from GPL sources, in any way, you must advertise that fact clearly, and extend an offer to the original sources plus your compilation methods to anyone who recieves this from you. This is true regardless of whether your work constitutes a combined work.
Then, if you are distributing a work that includes GPL parts and parts that you don't want to release under the GPL, you have to check specifically how the GPL parts are used. The relatively safe boundary is calling GPL binaries as separate processes, especially over a network - if this is the only way you are using the GPL code, it's probably OK to keep your other parts under an incompatible license.
If you are using the GPL parts any more closely, such as calling functions from a GPL library directly through an FFI, or worse, linking to that library, then you are almost certainly building a combined work and all of your own code has to be released under the GPL if you wish to distribute the GPL parts.
Even if you are calling the code only as a separate process, the amount and type of communication you use matters - if you are exchanging extremely complex and specific data structures with the GPL process, rather than just a few command line switches and parsing some yes/no answer, then your work may still constitute a combined work and have to be entirely distributed under the GPL.