The woman Chas is shown with is his wife Melinda.
Melinda was still alive at the beginning of the series, but died offscreen at some point during the early episodes. Her different appearance is just a case of Early Installment Character-Design Difference, combined with the fact that she may have been doing chemo and wearing a wig to hide her hair loss. The woman Chas is shown with is not his wife Melinda.
Melinda was already dead. The woman at the barbecue is just a friend, whom we never see on the show again because she's not a parent.