Given We reserve the right to reassign seats at any time, for operational, safety or security reasons. there is no cast iron guarantee so I am interpreting your question with a little licence, ie more highgood probability than certainty.
I think there is a high chance of persuading someone in a Standard seat to swap with someone who has a Priority with extra legroom seat, so one of you might pay £15 and be able to persuade whoever is sitting next to the other to relinquish their seat in exchange. To increase your chances, buy a Standard seat (+£8) for the other and choose a middle seat for them. Otherwise the Standard seat may well be next to a couple who will no more want to be separated than the two of you want to be together. Of course more expensive that plumping for £8 x2, but that might not be an option, depending upon when the seats are chosen.
Lucian has performed an amazing amount of analysis and blogged about it in an article titled How to Get a Free Seat on Ryanair. He suggests:
If neither of the above (pay or play the system) is chosen, or was chosen but did not work, there is still time for Plan B, though this is probably less likely to achieve the desired result. A low load factor should increase your chances greatly but that is unlikely, so your fellow passengers may not be in the best of moods for being herded like sheep and jammed into seats with low pitch. Plan B is of course as suggested by @Quora Feans, coupled with some incentive such as mentioned in @Dorothy's Comment, that might help overcome any grumpiness.
You would be advised to be careful with your selection however. Once one candidate has refused others may be more likely to follow precedent. So seek out someone who will at least understand why you are offering them sweets (ie is likely to speak your language – your friend might test them out in the language first, maybe "I'm not sure, is this 15B?") and, if you can, pick an aisle or window passenger seat next to one of your two allocated seats (to avoid trying to split others travelling together).