Re: Would you touch this with a 10 foot pole? Wood floor in a job that IMO should be
Within his parameters, I would think the steps would look something like this.
1. fix current house wall/porch roof flashing details so it no more water gets in. This is what I am assuming you mean by "water issues". If he will not consider this, then you are done, walk away.
2. rip out the slab/supports, and mitigate any issues with the brick veneer support found at that point.
3. build a wood deck floor using PT lumber that will work as a deck/porch floor even if he does not finish enclosing it. Proper header details at the house end, and probably pier(s) and a beam midspan, since the allowable span will will be less than the TJIs. Since it is a porch and not a deck, I think you might as you say get away with building even with the inside floor, allowing for 5/4, trex, or subfloor for later finish flooring, etc. as desired. If as you suspect he is not going to get around to enclosing it, mahogany, teak, or ipe wood decking looks nice if he's actually willing to splurge a bit, and is still good for an open porch.
Since he is talking to you "as a friend" rather than as a contractor, it sounds like he doesn't want to spend what he knows this will cost him. I would be very wary of not getting paid. If you are willing to do it to help him out and aren't concerned with the money piece (may or may not happen), make sure he buys the materials up front so you aren't out anything when it all goes pear shaped and he stops wanting to play with it and you can walk away from a disaster at least without any cost to you. If he is willing to actually pay you as a contractor, tell him he has to give you more control of the project to make sure it will be done to your standards.
I am curious what the finish grade is outside the block wall, is the current inside level with grade, or is it set down? If there are moisture issues under the floor beyond simply what is coming down the wall, then I would think some form of ventilation will be needed to make sure it does not stay that soggy wet under there. I didn't see any vents in your pictures?
If it is at the point where it is almost standing water under that space all the time (even accounting for proper roof details), I would think something like fix drains/downsputs/grading around house so ground water does not all run to/under porch, then fill to above the outside grade, some form of membrane floor to keep it from soaking in, and drains so water inside runs onto the surface grade outside. If everything outside is pushing water to this spot (house roof, porch roof/wall, ground slope, etc.) and he won't fix them, just walk away. About the only way to build above a swamp would be to use an aluminum boat dock...