There are very few good programmers out there. I suspect there is a high likelihood that if you have hired one of the few as your intern that the code he is looking at is in fact bad. I've seen much more bad code than good code in my career.
50,000 lines is pretty small. If it really is bad, it shouldn't take long to fix so why not let him have a stab at it? He's an intern, part of what he's supposed to be doing is learning things. Something that size should be fixable in a month if the things surrounding it are any good whatsoever and they don't have to be thrown out as well.
The last code I worked on that size that I claimed was bad and many longterm employees of the company where I worked at the time insisted was great ended up being about 1/3 of the original size at the end with significantly fewer bugs, additional new features, and had lower maintenance costs over the remainder of the years I worked there. Total investment...About 50% of my time for a month.
If the intern achieves the same type of results, great, you've found a stellar programmer you should hire full time. If not, your intern wasted some time and you throw away what he produced and get a new intern.