Visit HKSAR friend working in Shanghai - is her invitation good enough?

I plan to visit Shanghai for 3 days at the end of March. I have return tickets UK to Hong Kong already and will obtain tickets HK-Shanghai-HK soon. I plan to visit my HK SAR friend who works in Shanghai and stay at her apartment.

1) Will an invitation from my HK SAR friend be sufficient - for example, a hard copy of an email invitation? If OK, what details do you need from her?
2) Will printed flight E-tickets be OK?

Thanks in advance for your reply.