Send them a link to buy post wax lotion or gel and in future be stocked up and sell to every new client. Or offer to order it for them so you have the sale but get it to them quickly.
Bottoms come into contact with toilet seats which means germs. I once went to a spa after a bikini wax and contracted an unusual rash I’ve not had before on the parts waxed that were in contact with the seats, presumably from waxing. Went away just fine but it’s more noticeable as you sit on it, so more aware.
The hotness could well mean it’s a reaction or a burn, so suggest a compress and sudocream, but tell them you trust their judgment - if it’s a bit of a rash use the gel, if it’s clearly more of an issue to take care at home for a couple days using sudocreme and compressed but if it’s serious to go to a gp because you are not a doctor. I mean imagine if they had actually sat on an oven or caught an STI —-that’s an exaggerated example, to show that we don’t know everything and there will always be someone a bit dopey doing something completely unrelated and thinking it’s caused by us.
Now open your training book, find that contra-action and plan your operating procedures. When you tell a client the contra-actions they could possibly experience, what you are going to say and how you will sell them post wax gel explaining the benefits.