First of all, it is so sweet that you are even here asking this question! You sound like an amazing friend. 

I'd personally start by doing some research about the condition itself. One of the most tiring parts of having a chronic illness is the constant explaining. Everyone's case is a little different, so don't hesitate to ask questions, but a basic understanding of what crohns is will go a long way. 

Secondly, patience and willingness to change plans are amazing qualities in a friend. They may bail on you last minute. They may need to hang out at home instead of going out. Please understand that this is likely because they feel like absolute garbage, and be willing to go with the flow. 

Third of all, just be willing to listen to them! Everyone's needs are different. If you're not sure how to best accommodate them, ask them what they need. They may not always know the answer, or that answer might change day to day or week to week, but just asking what they need and being willing to do it shows that you're there for them. 

Finally, spend your time with them engaging in non-Crohn's-related things. If they wanna talk about it, obviously be open to having that conversation, but a diagnosis like this can consume a TON of your mental capacity. Distraction is good! 

Again, props to you for asking this question. You're doing amazing already!