Okay. I have a friend who used to play pokemon with me but she has since mostly lost interest. The few times i've been able to get here to WANT to play she ends up stopping cause she can't figure out what she wants to do. She doesn't like the idea of trying to catch them all since she only wants the ones she likes. She only owns Diamond and has beaten the elite four. She has limited experience with pokemon as a series (only played Diamond)...so, does anyone have any suggestions that might get her more interested in playing and keep her occupied? Any ideas are appreciated.

Well it's not always about having to catch them all. Try introducing her to something like competitive battling. If that may be to much try showing her some of the non main-series games like Stadium or Rumble.

I have to wonder why you're forcing her to play. If she doesn't want to, don't make her.

I think she needs to see other han Diamond. Try showing her HeartGold or SoulSilver, she might like them.

I actually kinda agree with jason here. It seems a liiil like you're forcing it unto her. :> If she wants to start playing it again she'll gradually come back too it~! =w=

Neo isn't forcing her to play. She already plays, but doesn't want to take up the goal of catching them all. She has only seen Diamond version and needs to be shown a bit more of Pokemon to see what else she would prefer to do instead of a goal that'll take time especially with 5th generation coming up.
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I recommend showing her Battle spin-offs/PMD/Ranger. Show her Competitive like Yoshi. I don't really know what else there is lol, I kinda forgot.

She could always check Blockbuster, when I was in the U.S they used to have DPPt when those were the games. Maybe they have HGSS there for you to look at.
EDIT: There is the TCG/Manga/Anime she could try out; TCG has cool art - the reason I buy it, Anime - ugh, Manga - I usually have to buy the spin-offs or loosely based manga, but go to Amazon and buy Pokemon Special *English name: Pokemon Adventure*

Introduce her to IV breeding so she can breed some strong pokes for me. You can also mention the Masuda method, breeding strong and or shiny pokemon will keep anybody occupied.