A friend has asked me to help her fix some net curtains to a PVA
double glazed bay window.

How should I fasten them ?

There is no wood, just plastic.

My inclination is to drill a hole and screw into it, but what is
likely to be behind it ?

Is it strong enough ?

What about making good the hole afterwards if it proves to be in an
unsuitable position ? - You can's just use filler and paint. It won't
match.