I don't know why everyone is reacting as if they know your friend better than you know her.
Plenty of people suffering from MH issues hate gifts (it would appear from this thread). But also plenty don't.
When I had a breakdown and was signed off I loved receiving gifts from people, exactly because they were treating it like an illness, which it is! Rather than some dirty secret never to be spoken of or acknowledged.
It's 100% to be respectes that not everyone feels like that. But OP knows her friend, so it's to be assumed she feels pretty confident that her friend would welcome a gift, just like many people with MH struggles welcome them!
I always send a book, unless I know that the recipient is struggling to read. (This can be easily acseryaibed through casual chat - "Are you managing to read much at the moment?" should do it).
For me, and many others, the ability to read and get lost in a book was a literal life saver when my depression was at its worst. (I vividly remember the specialist talking to me about all the books I had been sent in the hospital - he was amazed at how many I had managed to read, but it was the only thing I could do with my mind that made life bearable at that time).
Not everyone's cup of tea, of course. But personally I would ascertain whether she is reading at the moment, and if she is, that's what I'd send. Always.