Is it possible to help people doing uber tristram?

As already pointed out above, the short answer is "yes", so long as you're not in offline mode, obviously.

Beyond that, it depends on the actual situation you're in:
If you have your keys ready but doubt you can finish it, check the player services beforehand and find someone who can be on standby to bail you out if/when things get too tough for ya.

If you didn't plan that far ahead and are currently sitting in your uber run and just now learning that you can't do it yourself, a relatively reliable solution (depending on timing) is to either ask a friend or make a post here, asking for someone to come help you out. If/when you do ask here, make sure to only generally ask for help. Once someone replies whom you trust, add them as a friend and let them join directly (assuming friend join is enabled) or just message them the region, mode, game name and password. They can then jump in and save your keys from being wasted. Just make sure to not immediately post the game name and password when asking for help here (to avoid random lurking evildoers coming in and stealing your torch). Ask for a hand, include the mode (which you have here so good there), preferably the region and wait (hopefully/usually not too long).

Can't help you out myself, sadly, as I skipped this ladder season. But I'm sure someone else will happily come to the rescue here.