How Did Angela Know It Was George's Birthday?

After years of enjoyment, as I was re-watching it the other night, I think I stumbled upon a flaw--
The night that Angela meets George at his uncle's party, she walks in on him as he's practicing his pool shots. After a bit of flirting, George's uncle enters and insists that George telephone his mother to tell her of his promotion.
When talking with his mother, George makes no audible acknowledgement whenever she remembers it's his birthday. Angela is seen in the background several feet away, and unless empowered with superhuman hearing, she couldn't possibly have overheard his mother mention the fact that it's his birthday.
Yet shortly afterward, as Angela's coaxing George to join the party, she says to him 'Come on, I'll take you dancing on your birthday, "Blue-Boy".'