"Julia is Aamir's friend."

Translation: जूलिया आमिर की दोस्त है।

[SimricYarr]

Looks like dost changes gender depending on a female or male friend, yes?

[Zerenei]

Yep! Exactly

[shrimpbutwet]

Would ki become ka if we were talking about a male? Or is dost just a feminine word?

[vinay92]

Yes.
Eg: आमिर जूलिया का दोस्त है

Actually, दोस्त is a masculine word. It's use as a feminine noun like in the original sentence is not exactly grammatical (although it's commonly done).