It's easy to tell someone to "just stay postive" when dealing with ongoing illness or strife, but all the positivity in the world will not change their situation. When someone is going through a hard time, it can be difficult to to ask for help. Mpst people say “let me know if you need anything,” but what is even more helpful is, “What is something practical I could do to lessen your load?” If the person is not able to ask for help, gently suggest something you could do. Another helpful thing is to open yourself up for conversation. 

Here are some more supportive ways to show someone you're there for them:

https://themighty.com/2017/05/what-to-say-instead-of-just-stay-positive-when-offering-support/