In respect of my earlier post on this topic, I spoke with Rav Schachter and he suggested that as long as it wasn’t a מתנת חינם for the gentile, as per the תוספתא quoted by the מאירי, then a ברכה should be okay. He told me that a gentile had asked them to pray for her husband who was ill. She gave them $100 towards this end. This was permitted, because there was no מתנת חינם. Rav Schachter then told me a cute story about R’ Grossman from Migdal HaEmek when R’ Grossman was asked by an Indian Swamy for a ברכה.
Rav Schachter said that one side to be מתיר was possibly as a result of the ברכה the student will speak highly about me and I’ll get some benefit from that down the track. In addition, if I used a לשון which also blessed the student “to become a good בן נח” then this was definitely מותר and was a better proposition thaנ wishing he would not stay an idol worshipper 🙂