At risk of this becoming a flame war so to speak, I have to disagree with you; you present a strawman argument.

This is not outing him like you have it associated in your mind.

It is in fact the very definition of "outing":

• Does not have that person's consent

• Gives rise to issues of privacy, choice and hypocrisy (I really doubt OP would want her subs blabbing to her SO the intimate details of her proclivities until she's ready to have that conversation with them), and harm (not even OP has any idea how this guy is going to react)

• Sparks debate about what constitutes common good in efforts to combat [[mental health awareness and anti-shaming]]

  • Gentle reminder that homosexuality used to be considered "unhygienic" and was itself defined as mental illness until 1973

My argument is that sexual "perversion" (using medical lingo) and similar non-sexual BDSM proclivity, is inherent to the person in exactly the same way sexual orientation is inherent to the person, and therefor must be approached the same way.

Abusers and mental illness both thrive on silence

Same with homo/biphobia.

We're all agreed that there may be something mentally wrong with this sub and that, if there is something wrong with the sub, he needs professional help.

None of us are in any position to make that kind of judgement: we don't know the people involved, the full details of their situation, or have enough context to make a reasonable medical assessment; all we have are suspicions based on interpretation of what OP has posted (which are selected details of her own perspective).

If this is mental illness like you have it in your mind, I argue the domination aspect to it makes it more sensitive than other types of mental illness because of the stigma attached. E.g. Informing family and loved ones that you suspect someone is schizophrenic does not reveal anything about what they get up to in the privacy of their own bedroom. (The details of the basis for your suspicions should not be potentially embarrassing to anyone or, if they are, they should be easily obfuscated so as to still make the point. The same is not true of OP and her sub.)

If his wife does not aware she genuinely deserves to know what's going on, it is going to affect her too, and quickly.

Looking past the array of assumptions that statement makes and conclusions it jumps to, she probably should be informed and no, it is not OP's responsibility to do that.

A better approach is probably what others have outlined:

• Talk to him directly and state [OP's] concerns plainly

• Scale back or cease domination activities entirely

• Return the money if possible

• Encourage and help him seek treatment or counseling

  • Helping him seek treatment and talking about OP's concerns are what break the silence (instead of informing his wife)

• Encourage him to be honest with his wife, if he has been dishonest with her, and to seek her help as primary care-giver if appropriate (again, we don't have any details about the nature of their relationship)

• If there is evidence that OP's wife and family are, or are likely to be harmed (which OP may be able to glean from talking with him directly and honestly), I would suggest OP has moral obligation to work with a mental health professional (assuming OP is not herself a mental health professional)

  • The duty to warn seems to lie with mental health professionals and, as others have noted, at 19 y/o, OP is almost certainly insufficiently equipped to deal directly with her sub's mental health issues, if there are any

PS:

There are people in this world who would, given the ability to do it consequence free, kill us on sight.

Outing is never consequence free.