The most likely cause of your neighbor's tree problems was that the trenching to install the irrigation pipes cut the roots of the trees. While some tree root damage must occur when trenching, it can be minimized and the trees can survive. Do not trench on all sides of the trees within the drip line of the tree. If you trench radially and take care to avoid tree roots, you will minimize the number of major roots which are cut. Tangential trenching will maximize the amount of root injury.
Irrigation following installation of the irrigation system is important.
If roots were cut when the irrigation system was installed, it will be important to provide additional water until roots can be regenerated to replace those lost. After the roots have re-established, irrigation providing water to moisten the soil to a depth of two to three feet in the vicinity of the drip line should be sufficient. Irrigation every ten days to two weeks during the growing season should be sufficient. During the dormant season, irrigation should be needed once a month or less if the soil is moist. However, if there has been no precipitation, it may be necessary to irrigate during the winter.