Is it impermissible to take a portion of the money collected for a colleague whose leg was amputated and donate it to an orphanage, knowing that the colleague receives much assistance while the orphanage does not receive much, and that the intention was for all the money to be in God’s hand?

If the sister takes money as a charity specifically for her colleague, then she is not permitted to spend it on anything other than the intended purpose. If she spends it on something else, she is liable for it, as stated in "Mawahib al-Jalil" by Ibn Rushd and others. This applies if the charitable money is not from obligatory Zakat (definition); otherwise, it is only permissible to spend it on the eight categories specified by Islamic law.