Aiding a criminal is a serious offense that can have severe legal consequences. However, if you want to describe the concept without using the phrase "aid a criminal," there are many synonyms that might be useful. You could say that someone is helping a wrongdoer, assisting an offender, or supporting a lawbreaker. Other options include abetting a perpetrator, facilitating wrongdoing, or aiding and abetting a criminal. These phrases might be particularly useful in journalistic or legal writing, where precise language is crucial. Regardless of how you choose to describe it, aiding a criminal is a serious offense that can have devastating effects on both individuals and society as a whole.