Nikaah performed in jest in the presence of witnesses

I am writing anonymously on behalf of my friend, who seeks guidance regarding a sensitive matter.

Two years ago, when she was around 14–15 years old and her partner was 15–16, they performed a nikah with witnesses present. It began as a joke for their 4-month anniversary, but the full nikah words were said in front of 3 witnesses:

The friend asked: “Do you, [his full name], take [her full name] as your wife?” and vice versa

He replied: “Yes, I take [her full name] as my wife.”

She also said: “I take [his full name] as my husband.”

At the time, they did not fully understand the seriousness of their actions. They later consummated the marriage when she was almost 16 and he was almost 17, still unaware that it counted as a valid nikah.

Now, she realized the marriage may be considered valid in Islam, but she is unsure how to handle the situation given their young ages at the time, the accidental nature of the nikah.

They seek guidance on:

1. The Islamic validity of the nikah given their ages and circumstances at the time.

2. How to properly formalize or rectify any steps (including mahr, witnesses, or scholarly confirmation).

3. The correct way to proceed if they wish to continue the marriage safely and according to Islamic law.

Your advice would be greatly appreciated so that everything can be handled properly and responsibly.