Teamviewer, yes.
You install the full-fat version from the big green “download for free” button. They need to click the “join a remote control session” link underneath that, this will set up the server portion of the software without asking loads of questions. It’s worth getting confirmation from them of what the web page they’re on looks like before asking them to click anything, as there are a number of fake sites with similar URLs intended to catch out people who mistype.
If they ask you what to select from various options on the installer, they’ve ignored your request to click the “join” link and hit the big green button (which is pretty common). Ultimately this will still work but it’s less hassle just to cancel the installer and try again clicking join from the web page.
When they run the download, it’ll give them a 9-digit user ID and a 4-digit pass code. They tell you these, you enter them and off you go. If you disconnect say for a reboot, the ID should stay the same bit the pass code will probably change.