Any other free alternatives that are simple to use and secure?
I just use RDP, I haven't found anything that's faster or less resource intensive, possibly not as simple as, for example, CrossLoop but not that hard either.
More specifically, Remote Assistance - they send an invite, you use it to connect, they approve, (at least that's how I remember it working).
As far as security goes, isn't anything you get them to run going to leave an open port in their firewall after requesting access through it?
EDIT: Assuming it's Remote Assistance + Windows Firewall, Windows will close the port after use.