You say your mum understands what she's doing but honestly, I doubt it. Could you turn things round so that she tells you what she wants to spend money on but you make the transaction? So she would need to tell you to make a payment to this grandson, that would make it clearer what she's giving him. Have you asked what she spends the £200 per week on - can she account for it in any other way?
The grandson is definitely taking advantage. He's a grown man and knows exactly what he is doing - you need to stop it happening.
The grandson is definitely taking advantage. He's a grown man and knows exactly what he is doing - you need to stop it happening.