Ask an Immigration Solicitor. Get an Answer ASAP.
I am sorry to hear your friend's visa was refused.
The main reason is that your friend has not provided sufficient evidence to prove that she will return to her home country. It is unusual that a bond is being asked for. It is standard practice to ask for source of income. Your friend will need to provide evidence of her employment. You would need to write a letter of invitation confirming you will provide financial support and accommodation. You may also need to send evidence of your financial position.
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if your friend does not show proof that he is in employment in his home country and show the ties in his home country, the Home Office will no believe he will return.