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The emailer's accusations that this person "stalked" another therapist definitely needs to be explored and found out about in more depth. Hopefully the emailer is willing to give you the name of the therapist to whom this happened, and the former therapist is willing to talk with you. This kind of situation requires deep clarity of as many aspects as possible because there are at least two-sides to a story- and often more sides/opinions.
If you feel safe in this client's presence, I don't find it useful to 'confront' him. This may trigger him into a negative, downward cycle which may lead to other unsafe actions on his part.
Maybe ask him about his experiences with having been touched on his adductors (use cool, detached medical terms) and what happens for him, or even suggest that a male therapist may better suit his needs if the thighs actually need work. Otherwise, respect his wishes to not be touched there.
As for having another person in the room- what??! NO. Are there people around your studio? Is it near a public space? Hope so- better for your safety. Otherwise, in the business of massage, it's always good to be prepared to "lose" clients; the flow keeps moving.
Males becoming slightly or completely tumescent during thigh massage, while not common occurrence, isn't a symptom of a Chester Molester. We keep things covered, move off the thighs and ignore the natural reaction, politely allowing the client to pretend it isn't happening till the reaction disappears on its own a few seconds later.
As for the possible stalking. Since you work in a secluded spot, and you know in advance these suspicions about the guy, require him to provide personal references: employer, wife or parent and contact numbers. Less likely to act out if you could cause him problems in his personal life.
My personal opinion? I think he's just a pathetic guy with a humiliating premature ejaculation problem he doesn't want happening. Or maybe not, maybe one spot on inner thigh is his vulnerable spot?
However, absolutely don't accept as client anyone -- male or female-- you aren't comfortable with
I'll break down my response by sentence structure to hopefully make my thoughts more readable.
I should add that the anon emailer has advised me to use caution in terminating therapy with the client as this is what led to the stalking.