How Does Smoking Marijuana Affect The Heart?

Hello , my dad just had a quadruple bypass surgery and it actually was 5 grafts. my question is I know how much he loves weed , and he said he s never gonna smoke again, but I m afraid at some point his gonna feel so good his not gonna care and go back , especially since his been recovering fast . He s 62 years old and seems young but I m skeptical about him smoking even though he hits very small doses of weed at night. What can happen if he decides to take hits here and there, also considering he has no other health conditions besides the 3 blocked arteries they bypassed. Thank you.