Is it safe to boil sweetened condensed milk in the can to make duche de leche? 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