Act as usual.  If cheater suspects you know he might clear up his act  or become more cautious.

• Give your mate some space.  Tell him you are going away for a weekend or day stay nearby to follow where he goes or who he brings home.

• If you have fear of getting caught then:

send a friend he doesn't know to spy on him

rent a car

wear a disguise

wear clothes he wouldn't recognize and do not cause much attention

• Monitor the computer,  check what sites cheater is visiting, who is he emailing, chatting with etc.

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