Hello xLilianax
Maybe he had an update of his anti-virus software or firewall?
As he has connection with phone, even using wifi, it must be his computer. Usually it's firewall or anti-virus. He shall stop both, then start AChat and if it works, switch on firewall first, try again... if it still works turn on anti-virus and check the settings.
Please let us know if it's working again. Have fun with your friend