1983 Toyota Corona changing of wheels

My son bought the car - it had 13" rims/tyres. A friend gave him 14" rims. and he changed them to the 14". since then he has 13" on the back and 14" on the front. I told him he was crazy and that it may void your insurance if you have a car crash and it would make the speedo and milage reading inacurate. Can you tell me what other problems the 14" wheels will have and also the 14" on the front and 13" on the back
regards
a father pulling his hair out