A friend of mine tried playing Torb and was getting discouraged because of how easily the turrets die. It's tough for new players to keep them alive and make them useful apparently.
Any tips?
Don't go for max coverage. For example, a good turret placement is not one with 100% coverage of an objective.
It's not worth them getting destroyed so easily. Their job is to finish off kills, not completely kill people on their own.
Instead, put them in a place with a focused area of fire, with decent cover, making them harder to kill. Then make up for the lack of turret coverage by moving around yourself and dealing with enemies. Torb's gun is great, and the shotgun mode is particularly stupendous.
Also, if it goes down, just put up a new one. If you can fix it, fine, but don't worry if it is being destroyed.
EDIT: This is how to effectively use a turret:
https://gfycat.com/AptBadDuiker
Torb: ABF. Always Be Flanking