One employee prefers Google

Hey EvilTwinky100 

That is a valid concern you have about security however you need to tackle it from a broader perspective. I see some other people have already posted some great information but I'll add my personal touch to my reply.

The first thing you need to understand is why they prefer Google Docs for some insights. As Admin you need to put yourself into the user's shoes and think why do I prefer this?

• Did they never really use the MS 365 suite?
• Do they think the UI and functions are complicated or overwhelming?
• Is it just a preference/opinion?

When one of these answers is yes, you might need to organize some training moments to make sure they are comfortable using the M365 suite and show them the benefits and collaboration possibilities. This link can be useful: https://support.microsoft.com/en-us/training 

The second thing is as you mentioned security, This way of working is not that secure and makes it really hard to keep track of all the data that is flowing around in the company. As Tristan999 said you best inform management of this issue and educate them on the possible risks and possible legal results. Make sure they understand the importance of data governance and compliance.
The best thing you can do is flag it, if they don't care they don't however you raised this issue and you have it on paper that you did the best you could.

Hope my take assists in understanding the situation and hopefully you can sort out this issue!