How about having a small bolero made to give the sleeves, rather than adding them to the dress. . People might find that simpler without the worry of damaging the actual dress. It would also have the benefit that if the fabric is a bit different it won't look as wrong? She could also incorporate the bridesmaid colours into it a bit if she wanted?
It may be quite simple to add sleeves, if you think elastic in a hem round the top edge, which may be adjusted to give on or off the shoulder as preferred, but if she wants very slick tailored sleeves that is a bit more tricky.
Just some 'insta-thoughts'