In his answer to your question Dr. Monzer Kahf, Scholar in Islamic Economics & Financial Expert, stated,
Working in Islamic banks is permissible, no doubt about it and he should seek that kind of job.
If he works in a conventional bank where he does not write or sign an interest contract, his work will also be permissible but of course it is not the best job in the world, not like working in an Islamic bank or teaching Qur’an.
What is haram in Riba is only four: taking it, giving it, writing it and be a witness, other functions that do not make a part of these are not haram but they are only connected to it.
May Allah guide all of us to what pleases Him!