It’s a vague pronoun reference; there is no way to tell whose boyfriend cheated.
This sentence has a vague pronoun reference. With the her ahead of two girls’ names, it’s impossible for the reader to know if Alyssa was telling Charmaine that Charmaine’s boyfriend had cheated, or if Alyssa was telling Charmaine that Alyssa’s boyfriend was cheating, or if Alyssa was telling Charmaine that some unnamed third person’s boyfriend had cheated. To fix, the pronoun should be replaced by the noun it’s meant to replace. “Alyssa told Charmaine that Alyssa’s boyfriend had cheated.” Or, “Alyssa told Charmaine that Charmaine’s boyfriend had cheated.” Or, “Alyssa told Charmaine that Jessie’s boyfriend had cheated.”