Why do you think you should get to suggest to another adult who they should vote for? People do this all the time, its called canvassing, or adult discussion. I cant frogmarch her to the ballot station and direct her hand to which box she picks but i can certainly advise her which parties are definitely not going to be in her interests in all honesty unless, as above i am wrong and voting Tory is in her interests, but please feel free to show me where.
You've not given her an objective run down of the parties though have you? You've given her your opinion on the parties and told her who you think she should vote for.
As I said, if you actually cared about her voting rather than voting for who you wanted you would have cast your net a bit wider than just giving your own opinions. For example, if I was encouraging an 18 year old to vote for the first time I would encourage them to read party publicity, manifestos, as much press as they could get their hands on from the Guardian, to Huff Post, to Daily Mail. I would encourage them to read a bit of recent history to look at how the various parties had performed in office and then leave them to make up my own mind.
Although I suspect in reality they'd just tell me to fuck off and do whatever Buzzfeed suggested.
But anyway, my point is, you are obviously very partisan, so your 'rundown' of the main parties is not going to be objective or particularly reliable.