I wouldn't tell them straightforward. I would just send the evidence anonymously

I would let the cheater know that I know, and tell him/ her to confess him/herself. if they fail to do it I'll tell my friend.

Yes if u r truly their friend then telling them is the only option if u value the friendship n have respect n love for them

I will try to give a hint....

100%

Depends on ur closeness If ur not really close with them then it's better not say anything If u r close I say be sure first that it's really them And if ur sure and they are really cheating Then you should let ur friend know