We spread some diazonon and watered it in although I prefer Ortho's stinky powder stuff because it works so well but didn't have any handy. To create their passages, ants evacuate soil from around the plant's root zone, producing large air pockets. Without soil for root hairs to hold moisture, the root hairs dry out. Deprived of water, the plants wilt and die. This effect is exacerbated in small growing areas.
Plants growing in containers or in windowboxes are particularly vulnerable to ant activity.
Try watering the planter thoroughly until water runs out the drainage holes. This may help the soil re-settle around the roots. You may want to suggest your friend buy some Amdro. Unlike diazinon, this chemical will be carried back to the nest and will kill the queen. Diazinon kills only on contact.