My dad bought a can of pre-hopped Cooper's Irish Stout LME for his first brew. The guys at the store told him to just add a lb of DME and LME to pre-boiled water and then pitch the yeast. Is that true or does he have to boil his un-hopped DME to get the hot break?
No it doesn't have to be boiled. All that is done in the manufacturing process of extract. With canned extract being the exception, I prefer to add my extract to boiling water and bring it back up to a boil just for a second to sanitize it.
Canned extract should in theory have been pasteurized during packaging.
Once everything is dissolved, top off to your desired volume with cold water to bring the temperature back down. Ideally in the high 60's. Then pitch the yeast.
In this case, you don't need to boil the extract - just get the water to a strong boil, take it off the heat and mix in the DME (make sure it gets completely dissolved). The remaining heat should be plenty to sterilize the extract, but if you're nervous you can bring it back up to a boil for 10 minutes or so.