Yes he did and it is still leaking.
That sounds strange - if it's properly helicoiled, and the plug is correct, it shouldn't be leaking. Are you sure it's leaking from the drain plug?
If it's been helicoiled, I don't think the new oversized plug would work. It's only very slightly oversized, and I
think it's meant to cut into the original soft aluminum of the sump - which wouldn't be available with the helicoil in place.
Can he permanently seal the main drain and use the other two drain holes to drain the oil when needed? Will that be ok? I just don't want the the plug come off in motion where the tire will slip on freeway.
Yes, I see we haven't actually answered your question.
Don't do anything based on my input - hopefully the smart guys here will come by to give you a proper answer!
But... in my fairly unformed opinion: it's not ideal, but you could block off that drain if you had to. Here's why: I assume the "other two drain holes" you refer to are the screen access hole, and the crank chamber drain plug - the latter of which the early UCE's didn't even have. You'd leave a little bit of oil in the area above the primary drain plug, of course, but a lot less than early owners would leave in the crank chamber!
Here's a wordy post on oil draining I started awhile back, with lots of pics of the engine interior - it might be interesting for you to take a look to see (and read about) where the oil goes, and see the relatively small space above the primary drain plug which would actually hold oil if you sealed it. You can easily see where the original drain plug and the screen access opening are situated from the inside. It would take just a few minutes to read...
https://forum.classicmotorworks.com/index.php/topic,21436.msg239980.html#msg239980