That sounds mighty risky to me. I wouldn't smoke with him or encourage him to do so. If he says he can't smoke or do any activities because of his heart, that's not a good sign. I'm curious why if he says he can't smoke (or perhaps you're meaning cigarettes here), he's interested in smoking weed?

Look at it this way. We don't know the details of this kid's heart condition. But some of them can be bad. (The worst of congenital heart defects are generally fixed when people are babies or they don't make it to the teenage years or adulthood, but still, you never know. There are all sorts of arrhythmias and murmurs and valve problems people can have.) Weed smoking does indeed put strain on the heart. Probably it's less the weed itself than the action of smoking. It raises the heart rate and increases people's risk for heart attacks, arrhythmias, and strokes. But that's usually only a temporary increased risk. Still, with someone who has a heart condition--especially a mysterious one--why risk it? And if you did, what if something bad did happen? Seriously, would you want to play a role in risking that or being liable if something awful did happen? Would you want to be the one trying to give CPR or call 911?

If the kid is determined to try weed, he probably needs to eat it or vape it instead. And if he insists on smoking it, don't give it to him and don't participate in the process, either. Tell him you don't want to take that risk. Do a Google search on cannabis and heart attack risk and show him what you find.