When one of your friends need help, you will notice an exclamation mark next to his/her profile picture. Now visit their hospital and look for these exclamation marks in their hospital. Hopefully you have the right items to help them out! Maybe you have noticed three different colors:

Pink: A player needs help with cures for their patient(s)
Green: A player needs help with removing a harvested crop in his/her medicinal garden
Yellow: A player needs help with his/her Anti-Epidemic Center