In 12 hours you can expect to reach 120 and do about 18 uber levels if you somewhat know what to do.
The first thing of course are the first level challenge where ya all should split up and do them independently since that's at least 4 times faster, when someone (necro?) is done he can help the weakest (sin) player. Then ennead challenge, you can do that one together. Then the level challenge 2, again together. Then some easy ubers, then the black road.
https://docs.median-xl.com/ here go to quests, all Easy and moderate quests in the WESTERN KINGDOMS and KEHJISTAN should be do able with fresh lvl 120 self found chars. Once you have those charms you should be able to handle Triune and Judgment day. After that you need to gear up, probably Kurast3BA, Dcows, Fautz or Duncraig.
Check some guides here
https://forum.median-xl.com/viewforum.php?f=40 they have good info about ubers, write it together so you can present your friend what you need to do within 2 sentences, then go and rock it.
Maybe collect 2-3 builds for every char so they know what they should aim for, don't go for endgame builds here but rather the level up and farm builds, cheap gear etc.