All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
There is nothing wrong with taking a loan from your father to start a business unless the money itself is generated from unlawful sources. As for the chef working during the Jumu‘ah prayer and sermon time, it is a major sin and he must be admonished. The claim that he cannot embarrass the customers is rejected under the Sharee‘ah. Rather, he must order those who are obliged to pray Jumu‘ah to go out (to pray). Moreover, he has to make sign with such a meaning as well as avoid receiving customers whenever the time of the Jumu‘ah prayer and sermon draws near. Besides, he must adopt all possible means to fulfill this purpose. Let him know that any deal he makes after the second Athaan of the Jumu‘ah prayer is void based on the verse (what means): {O you who have believed, when [the Athaan] is called for the prayer on the day of Jumu'ah [Friday], then proceed to the remembrance of Allaah and leave trade. That is better for you, if you only knew.} [Quran 62: 9]. You should not give up advising him.
May Allaah The Exalted Grant you success, Support and Reward you with good.
Allaah Knows best.