The difference between an extract and plain leaf? To put it simply the extract is just a more potent, alkaloid dense form of kratom. What type of extract you have will determine how much you will need to take.
Something I'd like to point out in regards to dose is kratom listed as "15x" (or any x) does not actually mean that 1 gram will feel like 15 grams of plain leaf, so this is not a reliable way to figure out the dose needed to achieve comparable effects.
I suggest asking your friend the strength of your extract and what is an optimal dose to equal your dose of plain leaf. As far as wheter or not you take it orally, yes, extracts are to be eaten.