Is it permissible to marry a girl who is not religious, given that the father forced his son to propose to her out of fear for his health?

As long as you accepted the contract, even if unwillingly, it is valid. We advise you to hold on to your wife, if she is not openly committing sins and persistent in them, and to strive to teach her matters of her religion and help her obey Allah, in order to please your father and ward off anything that might harm him or affect his health. Perhaps Allah will grant you much good through her. However, if she does not become upright and treat you kindly, then there is no blame upon you in divorcing her.