The DM is allowed to arbitrate between when a character is or is not able to "provide help" on an Ability Check
If the Cleric is attempting to perform a Wisdom(Medicine) check—which I am assuming is the case, since regular Healing (i.e. any effect that expressly increases a character's current hit points) would bring back up an unconscious character with no check needed—the DM is allowed to stipulate that Help cannot be provided by a character whose capabilities in the check would be detrimental:
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 modifier—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.
—Working Together, PHB, pg. 175
Where this line gets drawn is a matter of DM purview. As DM, I would probably rule that a character with a negative modifier in their Wisdom(Medicine) score would be unable to provide help. Narratively, I'd justify this by arguing that with a negative score, your character is more likely to misunderstand a critical step of the process, and that their help would not contribute positively to the success of the check.
Not all DMs would agree with this though, and some DMs might allow a character to provide Help, regardless of their ability score. It's worth noting additionally that the language of this text implies that there is no requirement that a character have proficiency in order to help, unless the task in question requires proficiency. A Wisdom(Medicine) check is not such a task.