Two of the most common phrases I've heard:
Focus on your own journey.
If your teammates get better, you get better.
That is saying the there is a directly proportional relationship. Conversely, if your teammate regresses you regress. That is the definition of a directly proportional relationship.
I've seen the second prove true in every situation. The first can get you no where in terms of progress. If nobody intervened for this former D1 football player from hulking out on everybody, focusing on his own journey would have left him grappling by himself. You can can focus on your own journey, but if no one is leading the group as whole to progress, I can see a stall out. 
To actually benefit from videos, you have to have the type discerning eye a lower ranked person may not have. Many instructionals I see on YouTube miss details about how to fit it into game. Some I see are for situations that happen one out of a thousand rolls, making drilling them a waste of valuable time. Also, some moves are straight up garbage. You have to know how to deal with this when seeing videos 
But if you've already tried to explain to them why it is not the best use of training time and they persist then:
1. They are stubborn
2. They are stupid
3. They are lazy
4. You didn't communicate the issue clearly
5. Maybe some of these things are working for them
6. Maybe what they are being taught is not working for them
(I am sure there are others, but you get the idea)
Watching videos is in response to something.
People respond to incentives. There is a reason why these students are looking at videos and ignoring or placing less importance on class material.
If you want to help you need to think about the problem in a more general sense.
The original post moved from training partners to students and how you handle the situation would differ depending on your relationship to the other person. What rank are you and what rank are they? Perhaps the head instructor needs to make a general comment about the pros and cons of learning techniques from videos.
If you are close in rank to the other person they may just think you are jealous or that you are not "so advanced" that your opinion matters. I've seen both scenarios play out over the 14 years i've been training.
I would bet that boxing coaches sometimes have noobs come in --after watching clips of Prince Naseem or whoever on youtube-- and start trying to spar people with their hands down.
It's not that things are categoricaly 100% "wrong" and that NOBODY should ever do them, it's just that in all likelihood they are very probably wrong for you.
I mean, IF you have head movement and reflexes like a mongoose on meth, boxing with your hands down has some real advantages.
JosephConnolly - But if you've already tried to explain to them why it is not the best use of training time and they persist then:
1. They are stubborn
2. They are stupid
3. They are lazy
4. You didn't communicate the issue clearly
5. Maybe some of these things are working for them
6. Maybe what they are being taught is not working for them
(I am sure there are others, but you get the idea)
Watching videos is in response to something.
People respond to incentives. There is a reason why these students are looking at videos and ignoring or placing less importance on class material.
If you want to help you need to think about the problem in a more general sense.
The original post moved from training partners to students and how you handle the situation would differ depending on your relationship to the other person. What rank are you and what rank are they? Perhaps the head instructor needs to make a general comment about the pros and cons of learning techniques from videos.
If you are close in rank to the other person they may just think you are jealous or that you are not "so advanced" that your opinion matters. I've seen both scenarios play out over the 14 years i've been training.
When I was a white belt I thought flying submissions were awesome but as I progressed I fell in love with high percentage moves that worked. I feel like some people never outgrow the urge to be flashy. I teach a lot of classes and I also am asked to go and coach at tournaments the head instructor cannot make but I consider everyone my teammate/training partner. I am a brown for 2.5 years who has been training around 11 years and I have seen this problem amongst blues and purples. I will continue to try steering them in the right direction and hope they outgrow the urge to look flashy in the hopes they will seek out a style that works for them. The overall instructor has made comments many different times in a very polite manner but to no avail. I have no idea if they think I am advanced or not but I haven't given them any reason to doubt what I imho. Many times they have asked for my advice while training.
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I think it is a self correcting problem. If you try a technique for a while and it just results in you getting smashed, you'll abandon it. If it works, you'll keep it. People who want to get good don't stick with low percentage moves. If they find more joy in doing flashy, cool moves than being good....then that's OK too -- their choice, they are adults.
Also, I see a lot of people picking up the cool, flashy low percentage stuff from other students, rather than their own youtube excursions.
Trust me Randy I totally understand where you're coming from. I have this one friend who comes and says:
"I saw this awesome technique. First you..."
As soon as I hear the words "First..." I know the person doesn't understand the technique (not from instructors, from people trying to explain what they saw). I'll give them about a minute or two to try to get the technique out before I try to steer them into something a bit simpler.