Hi Obfuscious, here is some advice I have picked up in the past couple months. As with everything else, take it with a grain of salt as this is my opinion and doesn't apply to everyone.
First off, some background on ADHD. ADHD is developmental disorder that deals with the Executive functions of the brain and self-regulation. Dr. Barkley has a good video on this topic. Additionally the FAQ page on the side has more info. Understanding ADHD better will help you understand what might be going on with individuals with it. That being said, my personal opinion is that
those with ADHD need to be held to their actions. That shouldn't mean being cruel about a mistake they made. Instead, I means that the actions they make are their own and a part of who they are. They still need to do their work on time but be forgiving when they mess up.
Those with ADHD tend to have difficulties with inhibition, distractions, working memory, emotional control, and more. These difficulties diminish with medication but never fully go away. Now some actual tips to assist in the relationship. Remember that everyone is not the same and some of these tips may or may not apply to your instance. 1. If you ever need a task completed or something done, Be Concise and to the point with the instruction. Distractions and working memory make it difficult to remember long instructions and motivation tends to go down when it seems like a long and daunting task. I read that some have had success by having the ADHD person repeat it after words. 2. Additionally Write it down or send it in a text/e-mail. This will be more lenient on their working memory. Often times I'll remember the task (like go to the) but forget the details (what is needed at the store). 3. If you have a date or important plans Put it in a Calendar and create Alarms. I use a phone calendar with alarms. This helps the working memory again but is really for the person with ADHD. If you guys become close then you might want to share a calendar. You may need to remind them about the dates and jaxxly made a great post about this. 4. Motivation is often an issue, so Reward them when they do something correctly. I know that kinda sounds like pavlov's dogs but this classical conditioning can help. If they finish a project then give a back massage or a gift. If she needs a little bit of a boost then recommend going for some Ice cream after 20 minutes of work. 5. Verbal praise can also do wonders and can help with self-esteem issues. 6. It seems like a person with ADHD is constantly apologizing for breaking, losing, and forgetting stuff so you will need to Be Forgiving. Anxiety can be an issue and this will help relieve some of the stress they might feel. 7. To add to that, frustration and anger can often be misplaced. If they lash out then Be Understanding. This tends to be the most difficult part especially for a relationship as you may actual be hurt by something said and become angry yourself. So try not to escalate the anger into a fight. 8. After the situation has calmed down Have A Conversation. It is important to have open and honest conversation so that both sides can air issues that they might have. This talk needs to happen relatively soon to the event (within a day) so that neither side is holding a grudge.
Some of these tips are from personal expierence and others from reading/watching. My SO does a few of these tips for me and we have been together for ~5 years now. If anyone of my info is wrong please let me know.
TL;DR *Research more ADHD *Keep accountability *Be concise with instructions *Write info down (text, e-mail, stick notes) *Use calendars and alarms *Give verbal praise and rewards/gifts *Be forgiving and understanding *Have open and honest conversations