I have a friend that always lies to me.. I don't know when to believe her!

I have this friend that lies constantly. She'll lie and say she's sick to get out of work or sports she signed up for. She's admitted that she lied about being busy a day I asked her to hang out just because she didn't want to do what I suggested. Then that same night she hung out with another girl and lied about THAT later too. Awhile ago she told me that she had anger management, then just recently told me that was a lie. Also, she never talks to me to my face, it's ALWAYS by e-mail. This is really bothering me because lately she's been saying she suffers from depression and suicidal thoughts. And that is a very serious thing that I hope she would never lie about. But she's never told me this to my face. She tends to over react and make stuff up to get people to feel sorry for her. Right now, I'm just trying to be there for her because if she really does have issues then I don't want to accuse her of lying. I just never know when to trust her anymore and it's so hard to take her seriously. I have no idea what to do! Thank you to anyone who just listened to me spill this! Any advice is welcomed and appreciated. Thank you <3