Most of the brands I work with don’t hire a professional fit model. They hire (or convince) someone on their team, their sister, or a friend to be their fit model. This is totally fine as long as the person chosen represents the brand’s average customer and is up for the task.
Your fit model is the base of your brand’s fit, so it is crucial to find someone who is the same size, height, and proportions as your customers. Her taste doesn’t necessarily have to be the same as your customer, but it is nice if it is. As long as you can make accurate fit and design decisions for your customer based on seeing your design on the fit model, it doesn’t matter who she is.
When choosing a friend instead of a professional to fit model, you do want to set some expectations so everyone feels comfortable and prepared for each fitting. A fit model needs to be comfortable trying on clothes and having others evaluate them on her body. She also needs to be comfortable in front of a camera as you will want to take lots of photos during a fit session. You’ll want your fit model to maintain her measurements and weight fairly closely between fittings. It is normal, of course, for our bodies to change and fluctuate, but consistency is needed in this case.
When you are considering someone who isn’t a professional for your fit model, check that they will find this process fun and not distressing in any way. Not everyone feels comfortable being the center of attention.