It depends a lot on timing. If the WAllies attack during the invasion of Poland, then they could take the Rhineland with acceptable casualties; they would take losses, but they would gain a lot to make up for it. IIRC, during the Polish campaign the Germans had 20-25 divisions facing some 100+ British and French divisions in the West. If the WAllies had launched an all-out attack into the Rhineland, then they could have taken it fairly easily. Granted, they would have taken substantial casualties due to inferior doctrines and German defenses, but a 4-to-1 numbers advantage would probably guarantee success. Also, the added pressure in the West would mean that large numbers of German troops would have to be transferred from the Polish front to the Rhineland. This might keep Poland fighting for a couple more weeks, keeping the main weight of the Germans of the WAllies for a little longer. Once the Rhineland has fallen, Germany is pretty much doomed, since they will have lost an enormous chunk of their industrial production, and since they were already being greatly outproduced by the WAllies, this will spell their eventual doom. Also, the combat effectiveness of many German divisions will be greatly reduced since many of them will be thrown straight into heavy fighting after wearing out all their equipment against the Poles.
If the WAllies attack between the fall of Poland and the start of Fall Gelb, then they will likely get substantially less gains initially since most of the German Army will be available to fight them immediately, instead of off fighting Poles. However, the Germans will still be in the middle of refitting after the Polish campaign, and the WAllies are still outproducing the Germans, so they might still win. However, it's less likely that they will win in this scenario, since the numbers will be far more even initially and the Germans are much better organized and have a better combat doctrine, as well as lots of recent combat experience. If the Germans are forced into a battle of attrition, the WAllies will win, given time. If it turns into a war of movement, the Germans will likely win, since they have trained for that kind of war.