I agree that she has the more valuable animal and should get first pick, then alternate down. You can pick up a proven male pastel for less than $200, a breedable female spider is probably at least $1000 if not more IMO. So it would be quite easy for her to do this project w/o your male as your male isn't contributing anything particularly special or rare.

Personally, I'd probably only do a breeding loan if I knew ahead of time what all of the babies would be (e.g. pied x super pastel = all pastels 100% pied). This way you don't have to worry about what will hatch and don't have to worry about fighting over the clutch or anything.

(Note, I mean this as he's not special in that breedable male pastels are easy to come by, not that he's not a special snake to you or worthless or anything of the sort. Hope you don't take offense by my comment)