Is it permissible to partner with someone in a restaurant if the capital withdrawn by him for the partnership is from an interest-based bank loan, knowing that most of his other money is from his salary which he deposits in an interest-based bank, and that I will withdraw my money from an Islamic bank?

If your friend deposits his salary in an interest-based bank and takes interest, then dealing with him is disliked (makruh) but not forbidden (haram). He should be advised to abandon usury due to its severe warning. However, if his deposit is in a current account, then there is no harm in associating with him, even though depositing money in interest-based banks, even in a current account, is impermissible because it aids them.