a) Joel immediately went on to become Mr. Nicole Ritchie who is 2 years his junior. He doesn’t really have the reputation of serially dating young girls.
b) Joel posted this on insta which Hilary’s husband reposted which implies a camaraderie amongst the couples:

If not Joel… WHO?!
Let’s look at the next clue Hilary provides.
Hilary is referencing Jean-Michel Basquiat, a deceased Brooklyn painter whose work “Untitled” sold in 2017 sold for $110.5 million, the highest cost ever paid at auction for artwork by an American musician in a public sale at that point.
Who are known Basquiat fans?
Jay-Z and Beyonce own MULTIPLE Basquiats. A source of inspiration for Jay’s “Magna Carta Holy Grail” was Basquiat’s “Charles the First” Jay-Z’s past infidelity had been the subject of his own wife. ONTD, is Hilary Duff “Becky with the good hair”?? I’m here to say no. Simply put, Hilary’s lyric “She looks Like all of your girls but blonder.” all but eliminates Jay. Any good private investigator must look at all options.
Other famous Basquiat fans are Lenny Kravitz, Johnny Depp, and… Leonardo DiCaprio.
Unfortunately for OP & Hilary, Lenny has mostly been with women his own age (Lisa Bonet, Vanessa Pardis, and Nicole Kidman). I don’t want to spend too much time on Depp and feel comfortable excluding him based on Amber being older than Hilary. Which leaves Leo.
Let’s go back to the source:
Leonardo DiCaprio was born November 11th, 1974 which makes him a Scorpio. 🌚🕵️