We hear this reasoning all the time, and there are two problems with this. First, going to a DO school doesn't mean you're locked into primary care, it just may make the road a little harder and riskier. But declining a DO acceptance to continue an SMP ALSO makes the road harder and riskier. So I would consider the "fail" scenario. If he goes to the DO school and winds up not being competitive for his non-primary care specialty, he can still go into IM/FM/peds/a few other fields and make minimum $200k+/yr for the rest of his career. If he declines the A and stumbles on either his SMP or his MCAT, he's never going to be doctor and has to completely change his career path. One of those is significantly worse as a fail scenario than the other. And keep in mind, while he may be doing well now you never know when something unexpected could arise--he could have a family member get sick, he could have a personal life event that affects his mental health, he could find that he doesn't have the time to adequately prep for the MCAT while also doing the SMP, etc.
Secondly, getting into the MD school does not guarantee that he will be able to match a competitive specialty either. Plenty of MD students overestimate their competitiveness each year, reach for a specialty beyond their means, and fail to match.
Bottom line, declining an A for a nonguaranteed benefit does not make sense to me.