'Doing a Riff' is when one person claims to be madly in love with another person and enjoys telling everyone about it, any time. Then a few days later they have dumped them and are 'so in love' with someone else.
Person1: I love Jen so much! She's so amaaazzzzinnng. I just really, really love her.
***3 days later***
Person2: Hi! How are things between you and Jen?
Person1: Huh? Oh I dumped her. I'm in love with Angelina now. I love her SO much.
Person2: You're doing a Riff again!