In general, I would not feel comfortable exposing a camera/nvr through port-forwarding.
If it was a quick one-time thing and I was going to be around, I'd probably setup TeamViewer and let the person get to the camera via remote controlling my computer (with me watching everything).
If it's an on-going support thing <where I might not be available when they want to work>, I'd look into creating a VPN account for the person to be able to connect to my network.