Hello Gala,

Ants do not kill trees, but will nest  in a tree that is dying from other causes such as rot fungi, borer insect damage, storm damage, mechanical damage or diseases. Ants can  also nest in a section of a tree for many years without hurting the tree.

https://www.canr.msu.edu/news/carpenter_ants_nesting_in_trees_and_homes

If there is rotting wood, there is no cure, other than pruning out the rotted sections where possible. 

If your client can email you pictures, you can attach them here. We need a picture of the whole tree( taken from far enough back to see the tree top to soil line), a close up picture of where she sees ants or damage, and a close up ( from about 18-24 inches away) of a branch with several leaves attached, so we may be able to identify the tree and determine what the problem may be.

Pictures must be clear  and in good lighting, please.

Alternatively, your client can hire a certified arborist to come on site and give a complete diagnosis and plant health care plan. Find arborists by zip code here- http://www.treesaregood.org