We usually use "believe in X" to mean "I believe X exists" or "I believe X is true" For example: "I believe in Unicorns"= "it is my opinion that unicorns exist" "I don't believe in fairies"= "I don't think that there are any fairies in the world" "I believe in Islam"="I am a Muslim. I think Islam is correct, and other religions are wrong" "I believe in evolution"="I think that evolution is a correct scientific theory" We can also use "I believe in (person)" to mean "In general, I think (person) is trustworthy and capable of accomplishing his or her goals" For example: "Why did you give Tom that much responsibility?" "Because I believe in him." We use "I believe X" to mean "I think X is a true statement", and we use "I believe (person)" to mean "I think that the thing (person) just said is true" For example: "I believe that dogs are cuter than cats" "Bob said that he isn't guilty, and I believe him." So, in your example, both versions are OK. If you say "I don't believe in her", you are saying "In general, I don't think she's a trustworthy person". If you say "I don't believe her", you are saying "I think that her most recent statement is a lie".

"I believe her" means "I think she is telling the truth." "I believe in her" means "I think she will accomplish great things in future." Mom: "Who stole the cookies from the cookie jar?" Girl: "I didn't do it!" Mom: "I believe you." Girl: "Do you think I can win an Olympic gold medal someday?" Mom: "I believe in you. You can do absolutely anything you put your mind to."