You need an audio file converter or if you just want to listen to it, you can either use foobar or winamp (winamp needs a plugin though). You can convert it with something like DBpowerAmp.
Keep in mind that flac is a lossless format - so the file you were sent should be in it's original quality. MP3 is a lossy format so you are losing some data - most of which is not audible though. I personally go all lossless for both archiving and playback.