Your nephew (niece?) has section 3C leave which means that the conditions of her student visa continue to apply until the HO makes a decision on her application. This includes limited work rights.
If she considers entering the Skilled Worker visa route she will need to find a licensed sponsor who is prepared to employ her and assign a CoS to her.
Once she has a CoS there is no need to cancel (withdraw) her application for a Turkish Business Person visa, she can just
vary it but this must be before the HO decision
Please refer to
https://www.gov.uk/guidance/vary-your-i ... pplication
and p18 of the full guidance:
https://assets.publishing.service.gov.u ... drawal.pdf
Varying her application in effect means making a new application. When she starts the online application for a Skilled Worker visa it asks if she has a pending application. If she answers “yes” then she will vary the pending application as one cannot have more than one "live" applications at the same time. Fees have to be paid again but the fee paid for the previous application will be refunded.
A new Graduat Route will open on 1 July 2021 which will not require sponsorship for work for some graduates and this may be of interest to your nephew (niece?). Read the "Introduction of the Graduate Route":
https://assets.publishing.service.gov.u ... sible_.pdf