Everyone seems to have different ideas on this but personally I'm the same as your friend, I try to eat all 15 so that I don't feel deprived of anything and am less likely to binge. If I plateau I will reduce it to 10, if that doesn't work I'll take it down to 5 but never below 5.
But it's entirely upto yourself, only you can decide how many syns to have in a day. All I will say is that SW recommend at least 5 in a day - so don't restrict yourself to anything under this. A balanced diet should always include a few treats, everything in moderation is the key!