If the quest is not repeatable you cannot get the quest again.
If the quest enters an instance (rare in ESO) you may not be permitted in and some of those require them to be done solo.
There are also some quest areas that change after you complete the quests. If you did the quest and they have not you will often not seem them, but you can see their chevron. You will not be able to help in these areas either, but there are not many. The first quest area in Malabal Tor, Velyn Harbor, is an example of this.
Edit: but most areas you can at least run with them. BTW, they should not need help beyond getting an understanding of the game. A new player would be challenged with many WBs if solo, but most of that is knowing the game and not having some skills yet that will help them.