Unfortunately that's extremely hard to predict. The effects of Marijuana on cardiac rhythm is very poorly understood. Even with foods and environmental factors triggers are highly individualized.
Even in the case of Betablocker interactions with Cannabis generally most reports are anecdotal and seem to vary with each individuals response to the drug.
On the bright side; I don't know of any lethal instances of Marijuana affecting a patient. There's a risk of you feeling crappy but not a major risk of winding up in the icu or dieing.
That said; its worth noting that Marijuana does have other adverse effects. Despite what folks might try to sell you, chronic marijuana consumption, particuarly in the smoked form is every bit as bad for your heart and lungs as nicotine consumption.
In California patients have been given the option to use the drug legally without medical advice. I fully support this decision; however I think people need to be aware that you still need to consume the drug responsibly and be aware of the risks.
When the magical herb with seemingly has 1000 health benefits, which has 'never caused an overdose', gives you lung cancer or emphysema 10 years later; don't act say I didn't warn you.