Here's the rule for the Help action in the Combat chapter:
Help
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.
There is no requirement listed there for any proficiency needed in order to Help.
In the Using Ability Scores chapter, there's some more clarification on requirements needed in order to assist somebody with an ability check:
Working together
Sometimes two or more characters team up to attempt a task. The character who’s leading the effort—or the one with the highest ability modifer—can make an ability check with advantage, reflecting the help provided by the other characters. In combat, this requires the Help action (see chapter 9).
A character can only provide help if the task is one that he or she could attempt alone. For example, trying to open a lock requires proficiency with thieves’ tools, so a character who lacks that proficiency can’t help another character in that task. Moreover, a character can help only when two or more individuals working together would actually be productive. Some tasks, such as threading a needle, are no easier with help.
The requirement there in order to assist is that "the task is one that he or she could attempt alone". So, if they would need a proficiency in order to be able to attempt something alone, then they would need that proficiency in order to assist somebody else with it. It's pretty rare that a proficiency is required in order to attempt something, though, since in general there are only ability checks, not skill checks. All proficiency in a skill generally does is allow you to add your proficiency bonus if it's applicable to the check.
The requirement for proficiency in thieves' tools in order for a lock to a picked may be a property of the specific lock, such as the lock listed in the Equipment chapter. See "Do you HAVE to have Thieves tools in order to pick a lock?" for more on that particular case.
The "Jack of all Trades" only gives a bonus to your own ability checks, not to helping others make theirs. However, there's nothing about it that would prohibit or make it harder to offer Help either. But there's also nothing about it which would allow one to make ability checks that they otherwise couldn't make.
But certainly the DM would make their own call on whether any particular "task is one that he or she could attempt alone" based on the particular circumstances involved, and may want to take into account the "Jack of all Trades" trait when doing so if it may be applicable. But the key to whether helping somebody with their ability check is just whether it's something that could be attempted alone.
