Remote assistance, what can be done on a computer and other. The latter concerns any thing that has a web interface, or, more simply, it can receive an IP address in your network. In particular, video surveillance and intercom systems, access control. 

If a remote assistance program like TeamViewer is installed and there is normal Internet, you can do anything. With the exception of physical manipulations, the owner will perform the latter without any problems as instructed by the master. Here I would like to repeat once again about connecting to the Internet with a cable, and not via Wi-Fi. Although for simple tasks it will be enough.

The most important thing is that remote assistance saves a lot of time and your money. Of course, there is still the concept of trust in the master, not everyone wants to climb into their computer in this way. Well, in this case, do not store too valuable information. Or remove it in advance!

What can go wrong? Updating the router, this is where you need to be careful, because the owner may lose Internet. First, you need to find out the connection points to the provider, how the router's WAN port receives settings, sometimes static addresses, save in advance, is there any MAC address protection, sometimes it is cloned, that is, not the factory one.