Immune system help ????? !!!!!!!!!?

If i am overworked and my immune system is getting weak, what can i eat, take or drink to get my strength up and strengthen my immune system. Actually its for my friend........ She got 2 times in february??? .I am thinking of suggesting Dan Active but i dont know how potent that is. Please tell me stuff that are good. Stuff i can get at the store, not stuff online that i never heard of.... Please please......She looks weak and I dont like seeing her like this......