Hola, Vicente!

Here are a few points to consider in realtion to your text:

When my best friend borrow me some money...

You BORROW (money) from, so in this case you should say: 

My best friend borrowed/wants to borrow some money from me in order  to buy a car. What should I do?

Answer:
It's a decision quite difficult. >> It's quite a difficult decision. (word order)
I think the decision depending of depends on the level of friendship.
If you want to continue to be your her friend you should lend her the money.
You can lose money and you can miss friendship also lose a friend but it's necessary to take the risk.
If you don't lend her money you can lost friendship probably run the risk of losing a friend.
It's necessary to trust friends.

Friendship is more important than money.

BTW (=by the way), I agree with you 100%. Saludos!