There is a reason that the developed world has gotten away from lead water pipes. Even cold water can pick up traces and it doesn't take much to give you lead poisoning.
The Heat Exchanger may be made from copper but it was probably soldered together with lead based solder since it is not required to carry potable water.
The other concern is that if you are using a refurbished Heat Ex, it is probably impregnated with ethylene glycol which is very poisonous.
If you're dead set on using it, use it to chill the water/glycol that you use in an Immersion/Counter Flow/Plate chiller. Just make sure to test for leaks periodically.