بسم الله الرحمن الرحيم
It is mentioned in the Quran:
وتَعَاوَنُواْ عَلَى الْبرِّ وَالتَّقْوَى وَلاَ تَعَاوَنُواْ عَلَى الإِثْمِ وَالْعُدْوَانِ (المائدة: 2)
“Help each other in righteousness and piety, and do not help each other in sin and aggression” (5:2)
When your job is to guide him in company matters and bank loan is source of future finance, then obviously it involves interest, thus your help and guidance in this regard will be tantamount to grow an interest based work. So it is helping in sin and this is not right for a Muslim.
Allah knows Best!
Darul Uloom Deoband