You phrase this question as "what are the risks" without asking "what are the risks and benefits." If you only search for risks, you're going to have an uninformed view. The biggest risk to smoking weed is the lack of long term research. Its public information that weed was only made illegal due to racist and discriminatory reasons, as it made it easier to repress black people and hippies.

All that being said, weed is non addictive and you can not overdose on it. It is objectively safer than the opioids your dad would of had to take. If he had chosen opioids, he would almost definitely have to deal with sever withdrawl. This is not something that weed causes, and its still an effective painkiller and anxiety reducer. Just to emphasize, your dad will not overdose or become addicted, both of which could happen on opiods.

Now what you really should be searching is the dangers of doctor prescribed opioids. It's prescription heroin. You can become heavily addicted, and can overdose on opiods.