The first step is to talk to the home owner (who is presumably your friend) and tell them what happened. They may very well have a recommendation as to who they would prefer to have called in to repair this. At a very minimum they may be able to suggest a type of repair that they would be satisfied with.
Now onto the ideas for repair. If the dart made the hole in a flat spot the repair may very well be as simple as using a truss head sheet metal screw with a rubber washer under its head. Select the sheet metal screw that has an inner thread diameter similar to the dart hole size or possibly drill out the hole just a very small amount to adjust to a standard screw size. Also make sure the sheet metal screw is a type with threads that go all the way to the head. It also goes without saying that the screw length needs to be minimized. You so not want a long screw to poke out a hole on the far side of the radiator. For the rubber washer you may have to look at flat faucet replacement washers to find a suitable size.
When installing the sheet metal screw with the rubber washer be careful to not over tighten and strip the screw threads. This type of heater is often fabricated from a relatively thin aluminum material which can strip very easily. And use a hand screw driver, not some powered unit where over torquing would be almost inevitable.
Here is the type of screw I would suggest. Note the larger than normal size of the head.
In the end you offer to pay for all the repair parts and labor if that is requested by the home owner.