"Your friend is playing."
Translation:Dein Freund spielt.
If you mean the verb dienen = to serve, no.
If you mean the accusative masc. form of the possessive pronoun deinen: No, as we need to use the nominative form for the subject.
Freundin is a female noun, so you need to say "Deine Freundin". Note: Freundin means female friend so you can't use it in male specific context.