Engine getting hot and heater blowing cold air

My friend car temp. Guage reads hot and his heater blows cold air, and he says when he gives it more gas when driving it blows warm air out of the heater. I haven't had a chance to look at his car yet, we live in Washington State and the temperature has been in the mid. 30's. He just change the water pump recently and I'm wondering if the he didn't mix the antifreeze and water mixture right, with not a 50/50 mix or more water than antifreeze could this cause his problem? Do you think it may need a coolant back flush? Or do you think it's something else? Thank you for your time. I'm going to look at his car tomorrow.