So last session one of my players was enveloped by a Gelatinous Cube. While he was dissolving away, another player decided to slide/push the cube away with the result being that the enveloped player would stay in place, thus being freed as the Cube is slid away. As a DM, I stated that this sort of defeats the purpose of the cube's envelop and said the player would slide with the cube as essentially he is grappled and one with the Cube until he frees himself. The rules and creature description were kind of vague if not lacking in how this was suppossed to play out so I had to make a ruling. I was wondering what the actual rules are on this?