Maybe Lisa and Brad just realized they both wanted some sex that night. Maybe Lisa isn't all broken up by Brad dumping her right afterwards.
But let's play your scenario where Brad is the womanizing *****e breaking women's hearts everywhere he goes, while Dave is the nicest guy ever. I bet Dave consoled Lisa after Brad dumped her, then after the next guy and the next guy, hoping time after time she'd just realize the great guy was right in front of her. You know who the biggest a-hole in the story is, Dave because his long term friendship was based on just wanting to get in Lisa's pants. Brad and all the other jerks may have said some lies to get into her pants and it worked, but it was one night of lies, not months or years.