There are a few problems with this scenario - first of all, it is nearly impossible to prove. Owners laying down for each other in playoff situations are a LITTLE BIT easier to spot, but it's a very tough call in this type of instance. Second of all, if they are starting legitimate, ownable fantasy players, there is nothing you can really say. People make questionable lineup decisions for a number of reasons every week, and sometimes they work out, sometimes they don't. A lot of these issues are tied to name recognition and have a skewed view of matchups/strength of schedule anyway, so like I said, I personally think it's just too hard to prove one way or the other to do anything.
I would keep an eye on them in the future, and maybe if it is clearly happening in obvious situations where a win might benefit one person way more, or people are starting the likes of bench warming RBs and WRs then something can be done. Otherwise, I think you just have to stand pat. Hope this helps