Cannabis is still illegal in the States, for example, and Border Security/Immigration Services are able to ask a broad range of questions, including questions about cannabis use. They can even look at the traveller’s social media if they are suspicious. And with legalization in Canada, they may be more likely to question Canadian travellers about their use and be able to reject your employee.
If being able to travel outside of Canada is a mandatory part of the employee’s position (e.g. sales exec, technical consultant, etc.), employers will need to set more stringent policies on cannabis use for those employees. We don’t yet have all the answers on what legalized cannabis use will entail for employers, but this is one area to watch.