How can i get evidence of some one cheating without them knowing (by the way their really close to me)?

so I know this person but not going to say her name (cos someone i know might read this and tell her) shes married and has asked me to write several love letters to another man other than her husband, and shes asked me whever im angry or not, i said no but i truely dont know cos shes close to me too. But the person i think shes cheating on is like really really really close to me, and it seems he knows and is sort of hurt but this because he gave up so much for her and he sacrificed so much for her, and also had to put up with so many huge arguments!9 which had to envolve police cos shes like got anger problems) this woman told me not to tell him cos they would have an argument, and i have given him details of this already but he's rying to tell me to stop it cos of he doesnt believe me and tries not to think of it.i really dont know what to think of her anymore or what to do! help please!