I haven’t tried Artic Fox direct dyes but I think they’re similar to Splat which tends to stain quite a bit, if the hair is very porous underneath. So, I’d be cautious as a lack of fade would probably indicate damaged hair.
If it’s still very vivid, try using a direct dye remover to remove some of the colour and make it less bright. Malibu DDL is a good brand although there are a few others, inc. Joico, which I personally like. Look for products that are specifically for use with direct dyes and don’t need mixing with peroxide.
I also think it might be wise to lower your clients expectations regarding chopping and changing her hair colour after using these types of tint. The problem will be trying to get both sides to look the same when re-coloured. If the direct dye remover doesn’t do much, you could try a mild bleach wash but it will cause further damage so I’d be very clear about that. The clients I have who use these types of products and swap their colours often, tend to have very short/pixie type cuts where the damage will grow out quickly enough.