If it is aluminum, it "might" could be fixed. If the leak(s) are in the middle of the tube somewhere, what they'll do is clean it up really well using a solvent, then apply a vacuum to the core. Once it's got vacuum on it, they'll KEEP the vacuum on it, then apply their sealant if they can get to the leak(s), which sucks the sealant into the leaks and let it cure a while. Afterward, pressure test and fix other leak if there are any. If for some reason they can't fix the leak(s), they will give it back to you, ask you to pay the testing/diagnosis bill, and you get to buy a new one.
I've addressed quite a few heater cores and what I have noticed is that the #1 cause for leakage is failure to replace the coolant or in rare cases it just gets damaged physically.
Sometimes they are not repairable.