Is it possible to help people doing uber tristram?

I tried with Frenzy barb and can neither kill Mephisto nor Baal would be a shame to waste the entire Key set - is ladder.

I never did (help or get help) but there are a lot o Players Service Trades offering help with ubers
search.php?keywords=Uber&terms=all&auth ... mit=Search
Unfortunately only One is Ladder
search.php?keywords=Uber&terms=all&auth ... mit=Search

(on the 1st link there are some that do not specify if they are Ladder or Non-Ladder, maybe they help with both...)

Yes, you can invite someone to enter your game and help by asking here in the forum. If they accept you will need to message them your game name and password. Or you could invite a trusted friend to help as long as your settings are set to allow friends to enter your games.

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.

Thank you guys ~ was the later, tried with my Frenzy and got trapped in a house - together with meph. Found a friendly player who helped me. Was asking because when I left the game and rejoined it before the portal for Duriel was gone and I was afraid it is similar to ancients and the whole things resets or something.