It should be fine. 86 degrees is not that bad, many people have done cakes outside in much warmer temps! Especially if it will be in the shade. And the fact that it is chilled and covered in fondant will help too, fondant will protect the buttercream under it. If you are still concerned, the only other option would be to have them keep the cake indoors until the reception starts, so it is not sitting out the whole time during the ceremony too.