Supplements and vitamins are for those people who have a deficit. You’d need a doctor doing a blood screen to figure that out. I advise against just chugging down some stuff over the counter without knowing what she needs exactly.
The basics for good health: get enough sleep, get some exercise, eat healthy (veggies, fruits, fresh stuff, not a lot of prepared food, not much fat, salt, sugar). Get enough sunlight, both for Vitamin D and against seasonal depression.
And the mental aspect is often forgotten: have a healthy relationship that makes you happy; find a good balance between eustress and dis-stress (that is, neither too little nor too much work; work that is challenging not boring, but manageable); enough time for hobbies outside work.
If she’s depressed, that lowers the immune system for example. If she is getting no exercise, or doing too much exercise, that lowers the immune system. If she is around a lot of sick people, she might get infected no matter what.
One aspect of immune system is genetics and early childhood years, and there’s nothing you can do about it at all. So if she has done the above points, and still gets sick a lot, she either has to change her enviroment (get away from sick people or from the city, get a different job), or accept that she doesn’t have an immune system like Superman.
If she has specific problems - allergies for example - there are some alternative methods that work for some people.
If she bruises easily, or if her immune system has suddenly changed (she only got one cold per year five years ago, but now she’s had three and the winter isn’t over) she needs to get a full checkup from a doctor. Leukemia or another cancer or organ problem could be behind a sudden change. (I most certainly don’t want to scare you. I think it’s a remote possibility, and more likely it’s her current lifestyle - but you didn’t give any details).