Cheating on my hairdresser?

Ok, so I've been going to the same hairdresser for about many years and I'm perfectly happy with her. The issue I have is that she's currently not working due to planned surgery. I had my hair cut and coloured just before and didn't think it would be a problem going an extra couple of months in between appointments however my plans have now changed meaning I could do with a touch up before she's due to be back in work.

Do you think going to a different hairdresser for a one off appointment is ok? I usually have foils so just think I need my t-section or half head just to brighten up my colour and hide my greys but how do I do this when I don't know what colours my hairdresser usually uses?

Also, would a new hairdresser be pissed that I'm only going to them once?

I have anxiety so think I'm overthinking the matter but just need opinions. Alternatively, any suggestions for how I can brighten/cover greys at home without undoing the hardwork of my hairdresser?