Was Russell Brand cheating on his girlfriend during Forgetting Sarah Marshall?

In the movie Get Him to the Greek, Aldous Snow (Russell Brand) had
been going out with Jackie Q (Rose Byrne) for seven years. But in
Forgetting Sarah Marshall, which takes place two years before Get Him
to the Greek, he takes a romantic Hawaiian vacation with Sarah
Marshall (Kristen Bell).

However, in Get Him to the Greek, Jackie had admitted to cheating on
Aldous several times, and that he wasn't the biological father of his
son, Naples.