The following are ways a person can cheat his beloved.It could be either way.
1.Lying about your true age.
2.having a secret amorous affair with a third party often of opposite or even same sex without letting your partner know.Like double dating.
3.Giving out private information about your partner that you learned from him or her by virtue or your closeness without first telling him about it or using such to gain undue advantage of him in the situation.When this leads to loss of business then it is unfair.This is different from the civic duty of reporting crimes to assist the police as demanded by law
4.Witholding critical useful information or needed funds that you have with you under any pretext to your friend especially when you know he or she needs it.
Callous friends will have a bad conscience and will not be hurt but genuine friends will have sleepless nights as soon as they do such things.