Just go ahead and print it... should look fine from about 60 feet! Maybe use the "Enhance" button.
"Got an image enhancer that can bitmap?"
Seriously, though, in the real world that aint happening. There are some programs and methods to double and triple the resolution of images but you'll need to get to 3000px x 6000px (30" x 60" @ 100 px/in) starting with an image that is only 300x600px (about) -- that's a ten-fold increase and most certainly not likely to produce acceptable results...
Tell your customer you'll need a higher quality starting image.