Hilchos Lashon Hara chapter 9, section 2

Before many people, one may not even praise him a little because there will generally be those who agree or disagree or are jealous of him, and will disparage him because of the praise. If the person spoken of is known to be righteous and without evil, one may praise him even before an enemy or jealous people as they will not be able to disparage him, and if they do, everyone will know they are lying.