If you go up there to help, your added weight and the weight of your sled can introduce plenty of stress to the snowpack. Stay to one side or find a safe spot, but keep an eye on the stuck snowmobiler. Highmarking inherently involves being on steep slopes with potential avalanche hazard, so if you are inclined to play this game often it would be a good idea to learn more about stability analysis. Whether highmarking or not, consider your route carefully. The sides of ridges facing the winds are typically scoured, hardpacked, and safe, while the lee sides are often loaded with all the snow.
Cornices usually indicate that the slopes below are or have been loaded by the wind.
Sometimes by heading down and to one side at top speed, a snowmobiler can get out of the way of the avalanche. This is an advantage snowmobilers have over skiers and snowboarders, who have little chance of outrunning an avalanche. Large avalanches can run well over 100 miles per hour. If you are losing the battle, make as much noise and commotion as possible. At some point, you may want to part ways with your snowmobile.