I agree that is critical. 

I agree, my Vegas house keeps going up like crazy.

I agree with @Antoine Martel and wouldn't do this.

I agree with David, the boyfriend can act as a sort of guarantor for the girlfriend.

When purchasing property with a lease option and the intention of creating a sandwich lease option, or purchasing "Subject To" and agreeing to make the owners mortgage payments in both scenarios, would limited durable power of attorney get around the "Due on sale" clause and make a future closing easier?

Now if you use a private lender then it is whatever you can negotiate and get them to agree to.Banks and even hard money lenders want you to have "Skin in the Game" In other words they want you to have some of your own money at risk so if things go bad you are less likely to walk away.

@Irina BelkoferI agree with Grant!

You can’t expect him to agree to accept the terms/price in an agreement he hasn’t reviewed first, right?  

You must be in another part of BC which brings me to my next question - After you've spent the money fixing it up (which, most people will probably agree, you can assume will be at least 50% more than you've budgeted seeing as it's your first experience - and it's probably best if you calculate in a complete refurbishment of plumbing, electrical, perhaps roof or foundation - don't even try to do it piecemeal because it will screw you up in the end), after all that, will there be a GOOD market for resale? 

I would never agree to one as a seller, nor would I counsel any client to accept one.