Learn English – the saying or idiom or word that means when “friends” don’t need you anymore it seems like they don’t know you anymore

Charming comes to mind: very pleasing; delightful. (The synonyms of this word attest to its strength: beguile, bewitch, captivate, enchant, entrance, fascinate.)

He could charm the birds out of the trees.

Charismatic: possessing an extraordinary ability to attract. (Again, synonyms attest to its power: charming, appealing, attractive, influential, magnetic, enticing, alluring. With her striking looks and charismatic personality, she was noticed far and wide.) This word has been associated with Christian denominations, however, decreasing its appeal to some.

By virtue of his charm and charisma, Kennedy overcame anti-Catholic prejudice to win the presidency in 1960.

After you discover how he has turned on you, you might call him:

• a false friend [not to be confused with the linguistic term of the same name]:
  not faithful or loyal : treacherous <a false friend> (m-w.com "false", def. 4a)
• a traitor:
  one who betrays another's trust or is false to an obligation or duty (m-w.com def 1)
• a backstabber:
  [one who engages in] betrayal (as by a verbal attack against one not present) especially by a false friend (m-w.com def. 1)
• a double-crosser:
  to cheat or deceive (someone) especially by doing something that is different from what you said you would do (m-w.com def. 1)
• a two-timer:
  1: a double-crosser
  2: one who betrays (a spouse or lover) by secret lovemaking with another (m-w.com)

• disloyal:
  failing to support or be true to someone or something : not loyal or faithful to your friends, family, country, etc. (m-w.com)

• perfidious:
  not able to be trusted : showing that someone cannot be trusted : characterized by perfidy (the act of betraying someone or something : the state of not being loyal) (m-w.com)