You can use up to .5 oz of FO per pound of MP and soap. I would not go above that for Melt and Pour soap though.
Any kind of MP base is not going to lather as much as the detergent based bar or liquid soaps that you can get at the grocery store. (My hubby has the same complaint.) Different MP bases do lather differently, but they will all still have less than the store bought soaps. You can let her know though that this does not have any effect on how they clean you... more lather does not necessarily mean cleaner skin...
If she likes a lot of lather and liquid soaps I would try the liquid concentrate. You can either not dilute it, or dilute it only a little to suit her taste. This stuff lathers like crazy, especially in a loofah.