I helped to care for my great-nan who had alzheimers and dementia for 7 years. I've also worked with a lot of elderly and vunerable people. If their dementia is bad enough that they might change their minds mid service they will probably have a carer/family member present or making the appointment for them, who can deal with the issue. My Nan quickly lost the ability to make and remember appointments so we did it for her and usually had someone there with her.
If anything is going to happen it will probably be that they ask you repeatedly what you're doing ("Are we cutting it today dear?") Or as in the case of Nan ("Have I paid you already?") Smile, and gently answer their questions. Old people are usually creatures of habit and it's very unlikely that you'll be doing anything drastically new on them. If you have decided on a slightly different look with them if they say something about it during the service go with something along the lines of: "I think we decided to do........ to make it easier for you to manage / it will really flatter your face / this colour will look great on you etc." (I use that tactic quite often on normal clients whose hair didn't quite go to plan - spend long enough talking about why that little bit extra off has really pulled the style together / how that slightly warmer blond is not too harsh and really reflects the light beautifully etc
you can make most clients love whatever the result ends up being.)
If I may, the number one priority with dementia clients is: Dignify them and don't ask questions that require short term memory. If they repeat themselves pretend it's the first time you've heard it, yes you may have the same conversation 10 times over but if you don't say anything about it the client will feel happy and at ease. Refrain from questions like "What have you been up to today / what did you do yesterday / what did you eat for lunch etc." If they can't remember it will make them feel uncomfortable. Get them chatting about when they were kids and what it was like back then - you'll probably have endless things to talk about. It's also an amazing opportunity to hear first hand experiences of things like the war, being evacuated, etc that will soon be gone.
Whatever you do please DON'T say "You've already asked me that!" It will make them feel anxious and they cannot help it, so just go along with it, smile and enjoy the opportunity to learn a new way of communicating
From a practical mobility perspective I would be wary about agreeing to do a chemical service, especially if it's in their home, without having a carer present who could assist with moving etc. If you do agree make sure they are mobile enough to remain upright for the required time and are able to lean back far enough for the backwash / or have a chair that facilitates this. Most older clients will have a family member make the appointment for them so you can discuss this with them, and the consultation/patch test is a good opportunity to scout out the potential logistical issues.
