IMO, there really shouldn't be a NEED for Metformin on a re-feed day/high carb day, as your glycogen stores should be depleted from going low carb (assuming that is what has taken place prior). That being said, I don't see how it would have any negative effect being that it doesn't block the absorption of carbohydrates.
If you are truly glycogen depleted and need a refeed you should skip the metformin. As said above it inhibits storage. On a refeed day you are looking to store as much muscle glycogen as possible. Take with low/moderate carb days.