As Aiden and Eddie were working on the main title song for the album, Aiden was also making other songs on the side. After all, the album needed at least 5-6 songs.
One song with the 'The Boys' and one song with another female artist from the 80s was already decided. Unfortunately, they weren't able to find any artists from the 70s that were willing to work with him. It was also understandable as decades have gone by.
So, that era was eventually scrapped.
Instead, they went with an artist from the 2000s and that was easier. Grey was actually from that decade. His peak was then, so Aiden just contacted him and he agreed to lend his voice to a song.
Though Grey was already planning to retire so, other than lending his voice, he won't be able to promote the song a lot.