Do you know what exactly it is that bothers her about " pretty dresses" ?
Colours, like pinks or lilacs ?
Pattern like floral ?
Frills or decoration
Uncomfortable material
Not having the freedom of shorts of trousers
If you could find out exactly , that would help towards a solution. For example, there are lots of play suits that would he fine for a 7yo for a wedding.
www.newlook.com/shop/teens/shop-department/teens-pale-blue-bonded-lace-scuba-playsuit_516658445
This would work if it's shorts / trousers that she wants but not if she dislikes pastel colours.
culotte outfits here
www.next.co.uk/g752364s3
www.next.co.uk/g752362s1
Playsuit in non pastel colour ( red )
www.next.co.uk/g84228s2
Striped dungarees , would be fine with a blazer www.next.co.uk/g752346s1
Denim dress
www.next.co.uk/g75342s2
Denim quilted jacket and playsuit
www.next.co.uk/g6a192s6
I'm guessing it's not fabric or sensory issues as a shirt and bow tie with shorts are much less comfortable than a dress.
Does she actually want to dress in " boys clothes " ( which is fine of course ) or just dislike certain items of " girls clothes " ?
I think you need more information to enable you to address this in general, not just for the wedding. You want her to be happy in her clothes all the time.
What does she wear to school - trousers /shorts / pinafore / skirt - and is she happy with that ? That will give you a clue.
BTW I am have an 18yo DD who doesn't do pretty or frilly either. She's just gone to her high school prom in a very plain dress, minimal make up, no jewellery or fake tan and converse and she was very happy.
She know what style works for her and I hope you and your DD can work that out together.