I agree with others, find out if that dress holds a special meaning for her, or if she is just frugal. If it is the latter, then offer to take her shopping, spend a day together.
You could also invite some of their closer friends for a 'shower of blessings', where people instead of material gifts bring their well wishes, lessons learned, share some poetry or readings that are meaningful to them and suitable for the occasion. This could happen before or after the ceremony.
You could also invite to a sort of rehearsal dinner, as others have suggested, just as an opportunity for members of the two families to get to know each other better. Depending on the people involved and your mom's and her fiance's temperament, this could be formal or very casual, it could be just socializing or it could involve a sort of bingo or scavenger hunt game, where people learn different facts about each other.
There are all kinds of "icebreaker" games, that would lend itself to get to know each other better in a fun and relaxed environment, and the shared experience will forge connections and common memories.
Instead of shower gifts, people could pitch in and get them a partner massage and pampering before the ceremony as a gift; or in the week after the ceremony, for that matter.
I think what is appropriate really depends on them and who they are as people. Some just don't want to make a fuss for themselves but will appreciate it if others organize it, others just plain hate it.
Have fun with it, make it tailored to the couple and their families and what they will enjoy.