Caveat empteur, but how did you miss it in the pre-buy inspection? It looks pretty obvious from the pics.

I have never liked repaired masts. I dropped the mast on a P18 I had once and put a good ding in it. I bought a new tube and swapped out the parts, no question. Would not even consider using a compromised mast tube. Also there is the watertight issue, as you mentioned.
We have discussed on the forum how you CAN repair composite to perform and weigh as good as new. You should know that there is no way to do this with an alloy aluminum tube. Strength, stiffness and fatigue life will all be adversely affected, whether the repair is welded, riveted with doublers or bonded.

Jimbo