you have many choices.
1- try some emery paste on the allen tool head and see if it grips.
2 - weld a nut on this - be very careful as you dont want to heat it up too much. just one or two touches with an electric welder.
3 - hammer and chisel to rotate it counterclockwise.
- in options 1 and 2 you might have to weld a nut on it for further service or buy a new sump plug.
btw - how did the mechanic strip it? Whenever such a screw is worked on it is standard practice to clean out the tool grip area with a small wire brush or pick tool and then blow out with air - this ensures the allen tool grips fully.