"They are dating," the source says. "They've been spending a lot of time together and are having fun. He showers her with attention and is helping her move on from her breakup with Jamie [Kennedy]."
The I Know What You Did Last Summer actress, 31, may have met Asher on the set of her show The Ghost Whisperer when he taped a guest appearance for an upcoming episode earlier this year.
The source declined to confirm how the two got to know each other or when they began dating.
Hewitt's rep had no comment about the reported romance.
Hewitt and Kennedy – who broke up in March after a yearlong romance – continue to work together on CBS's Ghost Whisperer, and she says the situation hasn't been too terrible.
McCarthy and her companion, Jim Carrey, recently broke up after five years together. She and Asher were married from 1999 until their divorce in 2005. They have a son, Evan, 8.