As with my personal struggle, you can only help someone who wants to be supported & is willing to receive that help from you. It can be challenging to watch someone you care about keep making the same mistakes, or experience negative consequences due to their ADHD. (esacare.com)/top-3-benefits-of-an-emotional-support-animal-on-adhd-by-14-renowned-experts/
If they are truly ready to adopt some systems & strategies for managing their symptoms, the place to start is to learn at least the basics about ADHD. There are excellent books & several podcasts devoted to adults with ADHD. Both you & your loved one could read and listen to them & talk about what you learned.
The next step is to have a heart to heart about when & where they would like your help. Be aware of the potential for creating co-dependent dynamics where you are taking responsibility for managing his ADHD.
The ideal outcome is that you feel good about your level of involvement & your ADHD friend feels empowered.