If she wants a certain net wage, as it is a part time job you have to say, sorry I can't afford that. There is always going to be a limit for what people can afford, everyone sets a budget so all you can do is offer a figure up to your budget and hope you get someone interested at that salary level.
Lets say they have two jobs, each of which are 20 hours per week and paid at £12 gross per hour.
Job1
£240 gross per week
£12,480 gross per year
1100L Tax Code
NI: £530.40
Income Tax: £296
Employers NI: £602.78
Job2
£240 gross per week
£12,480 gross per year
BR Tax Code
NI: £530.40
Income Tax: £2496
Employers NI: £602.78
The Income Tax is the bit that changes and it changes a lot.
If they were to have one job, 40 hours per week, £12 gross per hour
£480 gross per week
£24,960 gross per year
1100L Tax Code
NI: £2028
Income Tax: £2792
Employers NI: £2325.02
Job1: £12,480 gross per year, 1100L Tax Code, NI: £530.40, Income Tax: £296
Job2: £12,480 gross per year, BR Tax Code, NI: £530.40, Income Tax: £2496
Total: £24960 gross per year, NI: £1060.80, Income Tax: £2792
So compared to having just one job, with two jobs they pay less NI (£1060.80 compared to £2028) and pay the same Income Tax.
It is their choice to have two jobs and by doing so they do save themselves some money. How you explain this to someone wanting a specific net salary I have no idea, it is really hard.