On a physical level, anything that helps with detoxifying would be helpful. Colon cleansing helps clear out the lymph glands. Have a look at Dr Schultz's American Botanical range of colon cleansers if you are in the US. They ship outside the US as well. A really inexpensive and well known cleanse uses bentonite clay and psysillium husks. I'm happy to email you instructions but you could also google it and find instructions on the web.

Exercise is great but trampolining is really good for lymphatic drainage (the body's immune system) and one of those small trampolines called a rebounder is also a good investment (as long as she actually uses it!)

Some types of massage also help - specific lymphatic drainage massage therapy. You would need to do a bit of research because it has to be quite a specific type to make any real impact on the lymph system.

Saunas and steam rooms are great too. Hot and cold therapies - anything that makes the blood move around the body will help with the immune system. A cheap way of doing this would be to have the shower on really hot and then cold and then hot and finish with cold.

Dry body brushing is a fantastic way of getting the lymph to move around your body. You need a get a body brush from a company that sells it for body brushing because it needs to be quite firm. Start off with the feet and basically work your way up to the heart. Sweep along the arms in firm long movements - basically you are trying to get the lymph flowing around the body rather than stagnating. Do this first thing in the morning before showering - if you wanted to take it to the next level you could then do the hot and cold shower treatment. Always finish with cold. Don't use perfumed body oils on the skin - pure (and inexpensive) almond oil is all you need.

Epsom salt baths - just put a couple of cup fulls of Epsom salts into the bath. This helps with detoxifying.

Green foods - Dr Schulze does a thing called Superfood which is so potent that it makes me go red in the face and tingle about ten minutes after drinking it.

At no expense whatsoever is a technique described in The Power of Now by Eckhart Tolle (highly recommended book). The key thing is to work on being in your body - staying connected with it at all times will have a big impact on your life. Keeping attention in the body as much as possible (noticing your breathing, feeling the different bits of your body) will keep you anchored in the Now. If you can do this it dampens down the running inner dialogue and so your thoughts and emotions don't overun you so to speak.

Feel your body from within. Say you are standing in a queue, use it as an opportunity to connect with your body. Basically being in the Now causes you to age less and is really good for your immune system.

The specific technique for boosting the immune system mentioned in the book is basically first thing in the morning (before your shower, green drink, trampolining ... LOL) when you are lying in bed or last thing at night "flood" your body with consciousness. Close your eyes, lie flat on your back and choose different parts of your body to focus your attention on briefly. First the hands, feet, arms etc Feel the life energy inside those parts as intensely as you can. Stay with each part for about fifteen seconds. Then let your attention run through the body like a wave a few times, from feet to head and back again. After that feel the inner body in its totality, as a single field of energy. Hold that feeling for a few minutes. Be intensely present during that time, present in every cell of your body. Don't judge yourself if you find your mind wanders. As soon as you notice yourself thinking, just return to the attention on the inner body.

If you were to do only two of these I would recommend the body brushing (inexpensive and easy to get into the habit of doing) and this visualisation exercise.

Lastly, I was told years ago I had a weak immune system - I dwelt on it for years hence my knowledge of lots of different techniques. The last thing I would add is that positive self talk really supercharges up the above mentioned suggestions. She isn't stuck with a weak immune system. She can change it because EVERYTHING can be changed. Good luck.