LOR from close friend with PhD from different field

[search the scriptures]

I am in the process of choosing who I want to write my letters of recommendation for the PhD programs I'm applying to in religion.  For 2 of the letters, I am obviously going to choose professors who I've had who's classes with that I have done well in and who know me well.  For the third, I was going to have one of my closest friends write it.  He has his PhD in a different field (Aerospace Engineering, from a top program) and can personally attest to my academic ability, work ethic and character.  I know people say LOR's should only be from people who's relationship with you originates either in academia (although we were high school classmates) or employment, and not from friends, but he knows me better than anyone else with a PhD who can write it.  Also, given the fact that his degree is in a difficult field from a prestigious program and that he's a recent graduate, I figured admissions committees would be welcoming of his perspective.  What are your thoughts about this?

[fuzzylogician]

Imagine you were sitting on an admissions committee, reading someone's letter that came from their best friend. The letter is exceedingly positive, highly recommends the candidate for admission, really makes the candidate stand out. 

Now consider this: 

• Letters from friends and family members are biased by their very nature, so the letter will be suspicious from the get-to.
• This author is in an unrelated field, making his perspective even more suspect. He presumably knows very little about the kind of research or even skills that are required in your field, and has no experience with them himself. He can't evaluate the quality of your work, and any opinion he offers on it can't be based on firm ground. 
• This author has little/no experience advising students, even in his own field, let alone yours. He can't compare you to other potential candidates in your field and speak to your strengths compared to them, nor to your potential to succeed in grad school and beyond.
• This author has never been your supervisor or advisor, and cannot speak to your ability to produce work within a given schedule, take criticism, improve, etc. He has also never been your teacher and cannot talk about your ability to pick up new material/skills, participate in class discussion, help peers, etc. 

However positive the letter, does it sound like it could be strong? I highly doubt it.

[MathCat]

I agree with fuzzy. This doesn't sound like a good idea to me.

[TakeruK]

I agree with the above--this will only hurt you. Also, high school classmates is not a relationship that "originates in academia". Usually, you want an academic relationship where the letter writer was in some kind of position to evaluate you (instructor, supervisor etc.), not a classmate. 

Also, you don't want an academic LOR to attest to things like work ethic and character. In my opinion, this is not the point of a LOR--it's not like letters of reference for employment. You want an academic LOR to be about your knowledge and experience and expertise/leadership in your field. Things like work ethic can be gleaned from the way your LORs say the above things and from your GPA. 

In your shoes, I would find a third professor in your field from one of the other classes you did well in for your third LOR. Even a mediocre letter from a professor in your field would be better than one from your friend with a PhD in a completely unrelated field.

[search the scriptures]

Alright, that solves that.  That's why this site is great.  Thanks for your insights.