In most cases, it's unethical. A best friend relationship is based on trust, mutual respect, and non - sexual affection. Once sexual activities like a blowjob are involved, it can change the dynamic completely. For example, if one person has different expectations afterwards, it can create a rift. Also, it may make other friends uncomfortable if they find out, and can even lead to the breakdown of the friendship group.
This is highly unethical. A marriage is based on loyalty and trust. What your wife's best friend has done, if it actually happened, is a serious breach of moral and relationship norms. It can lead to the breakdown of your marriage, cause emotional distress to your wife, and damage multiple relationships all at once.
If such a situation has occurred, communication is key. Both parties need to be honest about their feelings. They should decide whether they want to try and salvage the friendship or if it's best to part ways. It's important to respect each other's decision.