Is it ok to add a little water in car just til he gets home?

My son is at college and called to say his low level light came on and his temp went up in his car. He doesnt have coolant on him. He drives 15 miles one way. Is it ok for him to put enough water in it just to get him home? We gave the anti freeze here. Forgot to put new jug in his car. We try to make it a habit of keeping both our kids cars prepared for anything. Just forgot this time. If it's ok then when he gets home do we put in straight antifreeze or half and half? Thanks for all ur help. I am in pa so it is still cold here. Oh and we did have it flushed before winter started.