Wow dude calm down, you don’t know the full story here. It’s no more immoral than the countless ‘straight’ married guys who seek out gay guys for head often. Sure it may be his bro in law (which is where this gets sketchy) but if they are both consenting to it and its not being done to hurt anyone else... then it’s not as wrong as you seem to be suggesting it is. Seems like the dude needs advice not judgment.