Do not be one of those who shakes hands in a pledge, one of those who is surety for debts; if you have nothing with which to pay... (Proverbs 22:26-27)
Surety is another word for cosigning. To do so is to be totally responsible for the debt of another. Look at the wisdom and reasoning of this Proverb. If you are asked to cosign a note for the debt of another, you are being asked because they wish to go into debt and the lenders do not think that they have the ability to pay. Furthermore, you are 100% responsible for the debt. If the note is not paid, it goes against you as well as the first signer. Wisdom would be very careful in doing so. The only exception would be that you would be willing to pay the debt without remorse or loss of friendship.
More friendships have been lost and families put at odds over cosigning for debts than any other financial transaction. It is not only bad business; it endangers the best of relationships. Meet the needs of others as you are able, but do not enable them by cosigning for bad debt, and most debt is bad debt.