How to give encouraging words to an engaged unequally yoked couple?
My best advice would be truthful to them. I mean in honesty you do want them to be happy, and to have a good marriage, you will be their to support them regardless. So make sure they know that. I am not sure which one is the Christian but you should talk to them about it, if you already have then you have done your part. It is up to them to make that decision. From that point all you can do is pray for God's will and be their to show Christ's love to them.