Can i borrow 30k from a friend (for 3 months). No legal documentation etc. Is it allowed? Any tax implications for either of us. If yes, than whats the min I could borrow

I was planning to borrow some money from a friend. 

1. Is that allowed

2. Do we need to have a documnetation

3. Any specific tax / IRS rules we need to be aware of

4. Is there a limit i could borrow without worrying about these rules.