Slight of Mind by Gregory Sanchez

Volume five in the Rainbow Arc of Fire series is about betrayal in a relationship.

The main character, Greg, has apparently forgotten all about his partner, Paul, and his four closest friends, as well as about his own unique super-human abilities.

The others are forced to follow Greg to Europe with his new boyfriend, to discover who has caused this terrible change and how they might help Greg regain his memories of them and his abilities.