This is how it works in 5e:
You can lend your aid to another creature in the completion of a task. When you take the Help action, the creature you aid gains advantage on the next ability check it makes to perform the task you are helping with, provided that it makes the check before the start of your next turn.
Alternatively, you can aid a friendly creature in attacking a creature within 5 feet of you. You feint, distract the target, or in some other way team up to make your ally's attack more effective. If your ally attacks the target before your next turn, the first attack roll is made with advantage.
Second paragraph makes sense and we will not discuss it here.
First one is needlessly tied to "turns". What if you're going through an ancient library and looking for a forgotten tome? Seems that the help action would be applicable even if there are no turns to be counted.
When can I help? I assume I must declare HOW I'm helping, and the GM must find my idea reasonable. But HOW reasonable?
Should the Int 8 barbarian be that useful to the Int 20 wizard trying to find the tome? What about that Int 5 NPC that can barely read? What about the Int 14 cleric, should she be MORE helpful than the barbarian?
Can your cat familiar help the Str 20 champion to move a huge boulder?
|"Volo's Brazen Strumpets? Dear lord, can this brute even READ?"|
But you know me, I always prefer a simple mechanical solution to "the system is bad, but the GM can fix it if it gets ridiculous".
So here is the fix: the helper must succeed against he original DC divided by two to actually help.
Let's see some examples:
Find a forgotten tome in a huge library (DC 20): the barbarian with Int 8 will roll against DC 10 to be of any help. The chances that he will be useful are fifty-fifty.
Perform an ordinary task (DC 10): it only takes beating a DC 5 to help someone with an ordinary task, so in most ordinary circumstances two heads think better than one. Easy task? You can help 90% of the time even with no bonus.
Of course, the difference between cat, wizard and paladin looks too small, but that's an effect of bounded accuracy. I'd prefer using 2d10 instead of 1d20 for skills anyway!
Another way of dealing with this is group checks. But, as you know, if the Str 20 champion is trying to move a boulder, having a couple of Str 16 folks at his side can actually HINDER his chances! To fix that, try this post: Harder stealth (5e quick fix).