Can be done if it's a copper HE and if it's a tube. If it's leaking at the face where it meets the manifold, time for a new HE.
Lesson learned: No tabs in the skimmer and keep the pH close to 7.5 so it doesn't get eaten.
Additional Note: If you have a 2HP or bigger pump, a bypass should be plumbed to avoid pushing too much water through the heater, volume wise. A low head system with a big pump can push more than the heater should be asked to accomodate.