A friend is building a manifold and the copper connections he bought have a bit of tin (Sn) on it, supposedly to make the solder easier.
He is not going to solder them so the tin will remain in there.
Is it safe to use during the mash?
I'm not a metallurgist, but Tin is widely used to coat cans for food preservation, and is also present in the solder (leaded and non-leaded) used in water systems. With that in mind a small amount on the ends of the manifold should be no problem.
Tin is corrosion resistent, even in sea-water, just a little less resistant than passivized stainless steel, so it should be fine in the mash. If it does oxidize, the oxide film prevents further oxidization by acting as an insoluble protective barrier, much like passivisation layer of stainless steel.
I would leave it, but if you are concerned, it can be easily melted with a blow torch.