Not necessarily. There's no restriction on getting married in the U.S. The restriction is entering the U.S. as a visitor with the intent of getting married AND

Not necessarily. There's no restriction on getting married in the U.S. The restriction is entering the U.S. as a visitor with the intent of getting married AND applying for a green card. But if this intent arises after entry, there are no issues, period. Regarding these facts, your friend still should be able to travel to the U.S. Your friend should just be prepared to explain what they did while here and why they stayed as long as they and what they are coming to do now. At the end, it boils down to credibility and the truth. There's absolutely no legal reason here why your friend shouldn't be allowed in again.