According to TMZ, Robertson - who won the 16th season of ABC's The Bachelor and got engaged to the show's star Flajnik as a result - had dinner with Luyendyk Jr at Quiessence Restaurant in Phoenix, Arizona on Thursday (October 11).
Sources claim that Robertson and Luyendyk Jr were "all over each other" throughout the night.
Robertson and Flajnik confirmed their separation last week and thanked fans of the show for their continued support.
The pair - who initially broke up as the show aired, but later reconciled and renewed their engagement - blamed their split on "distance, time apart and [their] need to focus on [their] respective careers".
This week, Flajnik told In Touch magazine that the couple "really did love each other" but "weren't as similar as [they first] thought". He also revealed that Robertson was contractually obliged to return her engagement ring.
"Sometimes they make exceptions," he said. "[But] we've been together for ten months. You have to be together for two years to keep the ring."
The 17th season of The Bachelor will air in January 2013.