[General] She bought some cosmetic when in Hainan.

[Silverobama]

A friend of mine flew to Hainan to buy some cosmetic like eyeliners, lipsticks, etc because the things there are cheaper and with high quality.

I wrote a sentence to express the idea:

She bought some cosmetic when in Hainan.

Is the italic sentence natural?

 

[GoesStation]

She bought some cosmetics when she was in Hainan.

Is the italic sentence natural?

With my additions, yes. Cosmetic is an adjective. The noun, which is uncountable, is cosmetics.

 

[Alexey86]

What do you think of When in Hainan, she bought some cosmetics? I come across such sentences from time to time. Here's another one:

When in the shadow Cabinet, he called for a referendum on the Maastricht Treaty.
(express.co.uk)

 

[emsr2d2]

What do you think of When in Hainan, she bought some cosmetics? I come across such sentences from time to time. Here's another one:

When in the shadow Cabinet, he called for a referendum on the Maastricht Treaty.
(express.co.uk)

With "When in Hainan" at the start, I have no problem with your sentence. Of course, we would have to already know who "she" refers to.

 

[probus]

In both cases I would strongly prefer while to when. I can't say, however, that when is wrong there.