The Project_A is a decompiled version of another project (Project_B). But the original Project_B is not open source, AND Project_B never granted Person_A to open source the project.
You are asking a legal question (so consult your lawyer).
I am not a lawyer, but I am understanding that in the European Union, decompilation or reverse engineering of a binary software may sometimes be legal (e.g. for purpose of interoperability).
Clearly, the github sharer A violated the software license of the original Project_B.
That is your opinion, but what matters is what a court would decide.
(I am not a lawyer, and my understanding is that there are legal systems where this won't be a license violation; as an example, hash tables algorithms in Ocaml and in Rust are probably very close. See however in the USA the Google vs Oracle case, rumored to deal with 7 lines of source code)
A known precedent is Nouveau, obtained by reverse engineering of Nvidia binary drivers.
You may need to go to court. This is quite costly (possibly more than the value of Project_B) and may take years.
A related question is software patents. Legal framework is different in the USA and in Europe.
On the economical side, read the simple economics of opensource (and see also references in this draft report), and Weber's the success of open source book.
It could be more rational and beneficial (for both parties) to cooperate with Project_A.
According to rumors, some Nvidia engineers are legally cooperating with Nouveau.
D.Wheeler's sloccount utility might be used to estimate the economical value of Project_B then make a rational decision (cooperation vs. legal fight). Be aware that open source does not mean "no economical value": a lot of corporations are involved in GCC or in the Linux kernel and are making money by developing open source software. Be also aware that Debian or FreeBSD or Xorg are not made by unpaid amateurs, but essentially by a community of cooperating professional software developers. See Phoronix and LWN.
PS. In France, see APRIL and AFUL. I am member of both. Contact perhaps the FSF and the EFF and GPL-violations.
PPS. The important question is do you prefer to feed (that is spend your money on) lawyers or software developers.....