I left zbrush off the table for OP because many people don't own it. But yes decimating it in zbrush and then using a retopo solution would probably be best. Zbrush can handle something like that, especially if it's tris. Better results actually if its tris to quads from my experience. You could rent mudbox for a month for 10 bucks and do it too but the retopo solution there isn't as nice as ZRemesher. Actually if he can decimate into tris somehow, if it's not already, Instant Meshes can do it too but if it's quads to begin with it will probably throw a fit... it works best with reducing high poly scans with messy tri topology really.

On the topic of retopo Ghostman, you really need to play with Blender's Retopoflow add on... I've been having alot of fun with it. Clothes are ridiculously easy to make with it too.