Friends and Family
While in a perfect world everyone remains friends and family, in reality this is not necessarily true. Generally, in most cases, in-laws will side with their child and will distance themselves from the former spouse to alleviate and prevent any possibly animosity from rising. This can be emotionally challenging depending on each persons' situation.
Friends also feel like they cannot be friends with both people because there is a question and concern of loyalty. It's difficult to find balance and not feel awkward.