Lopez, 42, has been dating the backup dancer for about five months. The thrice-divorced singer-actress is the mother of pre-school-age twins.
UsMagazine.com said about 30 people attended the surprise party Lopez threw for Smart last Friday night at Cecconi's.
"Jennifer made arrangements beforehand and made some special requests: steaks!" a source told UsMagazine.com. "Lots of champagne and steak!"
Also on the menu were chicken salad, shrimp salad and birthday cake, the Web site reported.
The insider said Smart seemed "so happy throughout the night."
"He was very touched," the source said. "[Jennifer] really likes that guy."