It is unknown whether hash or cannabis can be dangerous for people with a severely damaged or weak heart but the increase of heart rate and lower blood pressure caused by being stoned can certainly cause potential short term problems especially if the person has a panic attack for whatever reason (thinking they're having a heart attack, wanting the effects to disappear or paranoid thoughts about specific things). This coupled with the already elevated heart rate seems like it can certainly cause problems for people with "bad" hearts, but it's still not proven as there's still been (of January 2020) absolutely no certain recorded deaths caused by only cannabis. There has been a couple reports of cardiac arrest, but most or all of these reports contain at least one other potential factor.