我個朋友想暫時用我個名去註冊公司

You can open a company in Hong Kong with $1 paid up captial and only 1 shareholder and 1 director. You can hold this 1 share on behalf of your friend in the form of a trust deed.
Most local CPA firms can arrange this for you.

坦白講，我會建議你唔好制，唔用得自己名開公司的原因通常係要做啲唔係點見得光嘅嘢，一係做犯法嘢，又或者瞞住公司做秘撈諸如此類，總之唔荒好嘢啦。

如果做犯法嘢，唔使講都知會有乜後果，你隨時做咗代罪羔羊，惹上官非都未知發生乜事。

就算唔係做犯法嘢，如果間公司爭人錢唔還，債主甚至乎收數公司會上門搵你追數，因為你係公司老闆，人地只會追你而唔會追你朋友。

幾好朋友都假，君子不立危牆之下，無端端惹禍上身就無謂啦。不過這是我的個人意見，俾你參考下啦。

Primarily you are liable for all the acts you made even you have an instruction to do for it under the law. So, if you are acting as a director of the limited company or acting as a sole proprietor of a firm, you are liable for all wrongful acts you performed even on the specific instruction from your friend. The outsider doesn’t have any information on this 'back' arrangement. Prima facie, you are liable but you can have the right to for action to claim back any damage, if any, from your friend as under the mutual agreement of trust or acting as an agent or attorney on behalf of your friend. It is normally a risky issue if you are also acting as a guarantor for the company's obligation e.g. bank loan etc..
You make out a trust deed for the share you held on behalf of your friend is only giving him a protection that he has the beneficial interest on the company but not for any impact or damages incurred, if any, on your performing acts upon his instruction unless a separate agreement is being made as mentioned above.