Q: Repairing my flat tire - 1999 Toyota Sienna

I found a big screw in the left rear tire a few couple of weeks ago. I took it to Big Brand tire to get it repaired but was told that it is not repairable because the location of the hole is too far out. I was told that I will need a new tire which will cost me $135 for the same exact tires that I have. Yesterday, my neighbor happens to noticed the spare tire being used on my van and asked me if I had a flat. Told him the story and he took a look at the tire which was placed underneath the car where the spare tire was with the screw still in place and he told me that it actually can be fixed. He's not a licensed mechanic, but he is good at fixing cars. He said that he recently had a screw in one of the tire on his truck in about the same area and he plugged it up and has been driving it with no problem. So, he helped plugged my flat for me and said that I should be good.

My question is...when is a flat consider not repairable? And is it really safe for me to continue driving my van?

My car has 119000 miles.
My car has an automatic transmission.