In my view, Econ student can always get finance position if they want to (they can always take the finance sequence in business school).
In terms of overall education Harvard is definitely superior. If your friend is 100% sure that he will do finance, then it doesn't hurt to go to Booth, but Harvard Econ will leave his doors open. I understand that at Booth you will still have option do some joint program with Econ department, but I don't think its all that common. The relationship between HBS and Harvard Econ seems to be stronger.
Also, in the end, you can do great work at both places, so the difference is reputation and selectivity. While Chicago/Booth is good here, Harvard is simply the best.