cpuminer and it's derivatives (including the cpuminer-gr fork) is GPL v2 licensed.
Modifying code, including the donation address is perfectly allowed. After all, thats excactly what the cpuminer-gr developer(s) did.
https://github.com/WyvernTKC/cpuminer-gr-avx2/blob/main/LICENSEThe only (marginally) infringing part is (if they did) not to include a written offer to receive the code of their patched version which alters the single line containing the donation address.