There are a myriad of reasons why people refuse to seek medical assistance. You are a good friend for caring...of course you want them to feel better if they are not well; however, the old cliché is true: you can lead a horse to water, but etc.

Speak with them candidly about your feelings / observations, and then don't bring it up again. They are adults and have to make their own decisions.

If their behavior becomes stressful / worrisome for you, remember your boundaries and do what you have to do to protect yourself