I was asked to write many character references for my sister. First...the person has to actually have character. It will be very hard to convince anyone, that a person who gets behind the wheel altered, has any character. I am not saying that to be harsh, but just to be realistic. You can only be honest. It will be very obvious, if you are stretching to make the person look better then they are. If this person is normally a fantastic person and truly screwed up, and you feel like this would never happen again, that has to be made very clear. There needs to be evidence in your mind, that the person is very remorseful, and that they have showed that to you. You should talk of how they are clearly a different person now, that they are very distraught over what happened, and doing everything it takes to change themselves for the better. It is always a nice touch, to include how this person impacts the lives of others. Do they volunteer anywhere, help anyone needy, give to others? Have they helped you in a time of need? Are they generally very responsible, reliable, loyal, and law abiding? If so, mention that.
I wrote a lot of what I did, because there was one time I should not have given a character reference. I knew that my sister would likely do it again, but I loved her, so I wrote the reference. She did repeat the crime. That time, it affected her unborn baby. I made a huge mistake giving a reference for someone who would re-offend, and they harmed someone innocent. If this person is likely to re-offend, please think very hard about writing a letter.