PS2 games don't need decrypting as they have no protection on them, dvd decrypter only decrypts dvd movies, protection wise.
The reason I asked about the chip was this, some older PS2 chips can't boot burnt DL backups, some can but the image needs to be patched with the toxic dual layer patcher before burning. Some of the later chips have an option in there menu to boot DL discs without the need to be patched first.
When your PS2 boots does it have any other text on the bootup screen? like mine has infinity come up. Better thought You could always ask your friend who installed the chip what it is.