This is much more complicated than you have described, and that is partly what is causing the difficulty. For a start you have not mentioned all the logical possibilities, or perhaps you just haven't given us all the information.
First of all, is there just one East loon or several? How big is the trigger area? If a second East loon spots a West loon before the chopper has been called, what happens then? What if he spots after the chopper has been called? Are these loons in one group or several? And so on.
However, there are at least two basic problems.
a) How to grab the East loon who did the spotting
b) How to control the chopper
Let's do b) first.
1) Chopper on hold waypoint
2) West detected by East trigger fires. The trigger is type switch and synchroed to the hold waypoint. The chopper moves immediately to his next waypoint, which is a move waypoint adjacent to the hold waypoint and has a delay of 15-20s (you'll have to experiment)
3) Chopper completes move waypoint and moves to its next waypoint, which is also a move waypoint and at the place that you want the chopper to go.
4) The chopper has, further down its waypoint sequence, the waypoints you want it to follow if the detecting loon is killed. If the loon is killed a switch trigger is fired and the chopper goes directly to that part of the waypoint sequence. Using repeating switch triggers and variables you can make a unit go to any part of its waypoint sequence as many times as you like. This is one of the great strengths of the ofp mission design engine and is much underappreciated and underused. Never use a script when triggers and waypoints can do the job.
Now, turning to a). As a general principle, grabbing the detecting loon out of several is not easy. In fact I have no idea how you might do it, not least because as soon as one loon detects an enemy he shares that information with the rest of his group.
If there is only one east loon then grabbing him is easy - just name him and we're done. If there is more than one then you need to decide what you want to happen. At the moment I'm not convinced that you are really clear in your own mind.