It depends on the nature of their relationship. If they are just good friends and there are no hidden motives, it could be okay. But in general, it might raise some eyebrows as it could potentially lead to misunderstandings. It's important for all parties involved to be transparent about their intentions.
In most cases, it's not very appropriate. Your wife is your partner, and there should be some boundaries. While they may be friends, going to a party together without you can create an uncomfortable situation. However, if it's a group event and there are other people around, it may be less concerning. But still, it's better to communicate clearly among all of you to avoid any potential issues.