They both need to get lawyers, because this situation could go bad very easily if the State doesn't agree to dismiss the charges. The State might very well prosecute this if they feel a crime was commuted, and your friends aren't married so they can't claim marital privilege.

Now that the state is involved, the State's Attorney will decide whether to prosecute. First degree assault is a felony and carries twenty-five years in prison. Second degree assault and reckless endangerment are misdemeanors, and they carry ten and five years in prison, respectively. Even, if both exercise their 5th Amendment rights not to incriminate themselves by testifying, the state might try to prove the charges, if it has other evidence. Unless each person is familiar with the criminal law, criminal procedure, rules of evidence, the prosecutor, and the judge, each should have a lawyer, so that the state does not take advantage of them. The law prevents one lawyer from representing both in this case, because the duty a lawyer owes to each client would conflict between codefendants or a witness and defendant.

If there is no evidence other than each defendant's testimony, then if they each assert the 5th, the state will have to dismiss. However, the state might dismiss one person's charges and grant that person immunity, in order to prosecute the other. The immunized person would the be subject to contempt if he or she did not testify. The court might summarily impose five months and twenty-nine days for contempt. If the state indicts for it, that carries ten years.

Each should get a lawyer. Each can invoke their right not to testify, but if the State dismisses one case and not the other, it can get tricky, hence the need for a lawyer.