In the US (I don't know if this is where you're interested), a school hiring your friend would have a bunch of added accreditation headaches if they have a non-math PhD teaching math courses. These are by no means impossible to surmount (the US system is probably more flexible than others for that), but the school would have the burden of proof; they would need to explain why this person is qualified to teach math classes, while no justification would be needed for a math PhD. Of course, having a PhD in math does not make you an expert in all areas of math, and an engineer might have interesting things to bring to the table, especially in a time of renewed interest in multidisciplinary programs, so there is a certain flaw in this double-standard system.

As I said, these complications do not rule out a position; in practice, I have no idea if this would be considered a big deal by most schools. Smaller schools are likely to be both more flexible, but also more bothered by accreditation issues, so it might be a wash.

[PS - The best strategy for your friend is to be upfront about it, both in the cover letter and also by contacting the schools directly to find out if it's a deal-breaker or not. I suspect the answer will vary from school to school.]