[frereubu]

I know this is about Windows, but just in case any Mac users don't know, there's a default app called Screen Sharing (in Applications > Utilities) that lets you dial into any other Mac user's computer if you have their iCloud username, allowing you to both see and control their screen. It doesn't work 100% of the time - sometimes it requires a tweak on a wifi router on the other end - but it's saved me countless hours on unproductive phone calls while helping my mother with tech issues on her iMac.