L.R.
Please don't pay for some expensive costume and wig she'll use only once! My daughter had to dress up as James Monroe (earlier period but still, long trousers and not knee breeches, hooray). We went to a good thrift store and I bought her a boy's plain black suit; a boy's lighter-colored vest that buttoned up in front; a plain white long-sleeved boy's shirt; and a woman's white scarf. Turn the collar of the shirt up so it comes up to her chin; wrap the scarf around like a man's neckerchief of the time (look at pictures online from the period). Tuck the scarf in the top of the vest. Black shoes (her own will do or buy boy's plain black shoes) or make false "boot tops" out of black posterboard that go over the shoes and appear from the ankle to the knee like a boot. Boots are NOT necessary -- city men wore shoes as well. And why a wig? Does she have long hair? If she does, put it up close to her head and get a cheap hat instead. If she has short hair, gel it close to her head -- men wore "macassar" hair oil back then to slick their hair down! And men were no longer wearing wigs by the 1850s and onward; wigs were really from the Colonial and early Federal eras so unless the wig is just to make her appear male -- there's no need.
The key things are having a vest or waistcoat and jacket, dark colors, and getting something around the collar that's not just a modern type of tie. Coat, vest and trousers are basic to almost the entire 19th and 20th centuries so that's easy!
Anything like that appears pretty much "old fashioned" and will look great on her. Let her do the shopping with you and let her research "men's 19th century clothing" online! Save dough and let her be creative rather than buying a costume.