Your friend will almost certainly have to improvise, and combine images from the Victorian era.
Taken literally, “Victorian” refers to a period of over 60 years. You only have to look at the changes in clothes fashions, social attitudes, entertainment styles, and household technology (for example) in the last 60 years to see that the idea of a homogenous “era” that long is artificial. For a modern piece of art, presumably the artist will want to convey the idea of “Victorian-ness” (Victorianity? ) to the modern audience, and our modern concept of Victorian combines bits and bobs from most of the 19th century.
For the unicycle, I think we have two sorts to look at: the entertainer’s unicycle, and the one ridden for sport - much as we can compare the circus unicycle and the MUni/Coker etc. today.
I’d expect the entertainer’s unicycle to be built as a one off, with quite a choice of wheel diameter because they would have used solid rubber tyres. the wheel rims would have been very narrow by modern standards. The spokes would have been radial (i.e. going straight out from the hub to the rim without crossing) and the pedals would probably have been quite heavy, with discs on the outside edge.
The enthusiast’s sports unicycle was, I believe, a derivative of the “ordinary bicycle” (aka “penny farthing”). I’ve ridden a penny farthing (a fantastic experience - if ever you get the chance, seize it with both hands) and the small back wheel hovers, or just skims the ground as you ride. Some people took this to an extreme by removing the spine and seat of the cycle, leaving only the wheel, forks, cranks, pedals and er… handlebars - not the seat. (You occasionally see modern novelty unicycles on ebay which have handlebars instead of seats.)
So for the style of the unicycle, look at the many pictures of penny farthings that are readily available. Take the styling features and blend them into a unicycle.
As for the costume. Look at what Victorian penny farthingists wore, or look at what Victorian circus performers wore. Allow for the practicalities (breeches and tight stockings, rather than baggy trousers) and a bit of flair (suitable hat and stripey waistcoat) and you’ll be halfway there.
There is an (American?) firm called Rideable Replicas that makes a modern “replica” of a big wheeled unicycle. It is a stylistic replica rather than an authentic one, but could provide inspiration.