Is it? I am talking about metal poisoning not the explosion risk, will the can leak dangerous metals into the dulche de leche product when boiled or is it safe to eat? (I am hoping it is because it is very convenient but if its not what to do safety first)
Yes, it is perfectly safe. Nearly all if not all canned foods sold commercially have been processed in retorts (industrial pressure cookers) to kill microrganisms The metal from the can is not going to leach into the food contained in it
I don’t mean this in a snarky way, but what’s wrong with just dumping it into a pan, double boiler maybe? I know you have then created one more dirty dish but that eliminates the possibility of any off tastes or metallic tang in the milk.