Sorry about your grandmother, but what would be the point of texting him (assuming it's just one person and you have his number).
Depending on what you say, it could land YOU in trouble if he were to report your threats to your carrier or US authorities.
Considering he's a scammer that makes his living from swindling the elderly, I doubt he would care too much if you were to 'tell him off.' If anything, it'd probably just make him laugh since he'd know there's virtually no chance of you flying out there to actually do something.
If there's nothing the local police or banks can do for you, you'd be better off moving on and using your energy to inform others to not get hoodwinked by the same scam.