The event, which will largely be “low-key,” will take place on Sept. 15 at the Foster residence.
Foster’s wife, Katherine McPhee, went to the same school as Meghan in California.
A source spoke to Sunday Mirror, saying: “David wants to make it special for Harry but also keep it casual and low-key with some fine wine, great food, and intimate company.
“He is setting aside the fire pit at his home plus the whole outdoor area, which security will cordon off.
“The small gathering will also include his daughters Erin and Sara and their husbands.
“They are around the same age as Harry and Meghan.
“David has been helping to connect the Sussexes so they can create a group of close friends and feel more comfortable in LA.”
His wife, who is a singer and an actress, has previously been quite vocal about her husband's friendship with the Duke of Sussex.
“David has got a really beautiful relationship with Harry. They’re so cute, like father and son.”
Foster had even helped Harry find a home in Canada’s Vancouver Island when the couple had moved there after parting ways with their royal duties.
McPhee has much to say about her husband as well. She says that he “loves to help the people he cares about” and has a big heart.
She added that she had known Meghan her childhood, and that “it’s nice: that her husband is friends with Harry.
The Duchess had also marked her last birthday when she turned 39 in a private ceremony with her husband. Marking the day, Harry was said to cook a three-course meal for his wife while Meghan’s mother Doria Ragland took care of young Archie.
"They spent the day as a family and in the evening, Doria looked after Archie so that Meghan and Harry could enjoy some couple time,” a source close to the family said.
"Harry cooked Meghan a three-course dinner, but Doria helped him with the preparation.
“While Harry has become a better cook since marrying Meghan, he still has a way to go!"
Wishes from Buckingham also flowed during Meghan’s birthday.
"Wishing The Duchess of Sussex a very happy birthday!" wrote the Queen, sharing a picture of Meghan with her during a 2018 visit to Chester.
There were also wishes from Prince William and Kate Middleton: "Wishing a very happy birthday to The Duchess of Sussex today!"
Amid other developments for the couple, the new biography Finding Freedom by royal reporters Omid Scoobie and Carolyn Durand, which shares exclusive details from the lives of Meghan and Harry, is set to be released on Aug. 11. By Harper Collins.