Haven't done taxes in 15+ years

The header doesn't apply to me but I'm trying to help a friend get their life together and one of those things entails filing their taxes. They said they haven't done them in 15+ years. Personally, I go through my financial advisor to do my own. Where should we start? What companies and services would you recommend? My understanding is that he is likely owed money and would have already been penalized if he owed on his taxes and hadn't been paying them. Any advice or direction would be greatly appreciated!