Advise

My highly respected work collegue whom i work with every day, her partner of 1 yr lives next door to me. Mulitple times at night time i have seen his ex over there in his bedrm and the shadows on the blinds make obvious that hes cheating. This work collegue is also a good friend, i want to tell her but obviously kinda arkward to say hey btw ur bf n his ex r fuking..then il get questioned how and why i saw then wen she says somethin to him itl be arkward neighbours lol.