Very bad idea.
For one thing, the border will likely have tighter security as it's a big day for the US.
For another thing, neither of the items mentioned proves Canadian citizenship.
If he tries to use the credit card as ID, depending on how grumpy the officer he's dealing with is it could be contrued as a bribery attempt (not hard to get a cash advance on the card from the machine in the office).
The only items the prove Canadian citizenship are birth certificate, passport, or citizenship papers. That's it. You have to have one of those - especially on the 4th of July - or chances are you will be turned away at the border.
The other concern would be getting back into Canada - just because you have the DL and health card doesn't mean you're a citizen. If you can't prove your citizenship, they don't have to let you back into the country.
Case in point: My DF. He's a US citizen, living and working in Canada, with landed immigrant status. He is not a Canadian citizen. He has a Canadian DL and health card. When he goes across the border he still has to show ID showing his citizenship. On the way back into Canada he has to show his landed immigrant card as well.
After all, I don't think you have to turn in those items when you move out of the country. For all the customs agents know, he could be a foreign citizen that was workingin Canada, lost his job and had to leave the country, and is trying to get back in and claiming he's a Canadian citizen.
In the past 3 years I've driven across the border many times. I've always been asked for ID. Sometimes the questions are pretty basic, sometimes they're a little more indepth. But I've always been asked for correct ID.