Well, the only thing I can think of is to phone Crimestoppers on 0800 555 111, and tell them the whole story. They have the power to tap phone lines, and record texts and conversations. You can also email the police with the full mobile phone number of this person.
One thing I do know about drug dealers is that they have code names and words for drugs and places, so it probably would not mean anything to the layman, and the police would have to decify any texts.