Topic Healer and meters
Meters are incredibly useful in assessing healing. However, you can't JUST look at the raw numbers.
You need to understand the class in question, how they perform on specific fights, the spell breakdown that would be ideal for any given encounter, dispels, damage they take from unnecessary sources, deaths suffered that could have been prevented, whether or not they did their job if assigned to heal specific people, how they handled their CDs, how their gear level compares to other healers in your raid....
Raw numbers give you a very incomplete picture. Understanding all of the above will give you a much better idea of whether a healer is struggling or not.
That said, if a healer consistently performs below your other healers in a challenging raid difficulty for your item level, there's a very good chance they can increase their performance.
"Why, if no one died?" you might ask. In a very casual guild, that might not matter at all. However, if you're doing raid content that's challenging your raid, every bit helps. A better understanding of your own class (and other classes! Honestly, a great way to improve your raiding is to understand how other classes work) will make killing new things much easier.
For instance, check to see if there is a Disc Priest in raid, first.