Praising a person in a situation where someone present is likely to temper such praise with criticism is another form of avak loshon hara. It if forbidden to praise someone in the presence of anyone who is known to dislike him, as it is common for such people to respond by mentioning the person’s shortcomings.
Therefore, it is wrong to praise someone in the presence of a large group of people, since there is a reasonable chance that at least one person in the group dislikes him.
Likewise, it is forbidden to praise a businessman in the presence of his competitors even if they claim to harbor no ill will towards him.
Excessive praise should always be avoided for it can often cause people to respond negatively even when they have no particular dislike for the person being discussed.
Sefer Chofetz Chaim, 5 Cheshvan, page 118