I would recommend her aunt sit her down and TALK to her - like an adult - about sex and her options for BC - and the importance of BC. Not just pregnancy (which is all most teens care about) but HIV and other STDs. And that STDs can be passed via oral and anal sex too - most kids don't seem to "get" this concept, and oral and anal has been on the rise in teens - more so younger teens as an alternative to sex.

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Anyways, take her out to lunch or something and just talk to her about her options - not lecturing though. Take her to Planned Parenthood for a checkup - if she is sexually active, she now needs a yearly exam and PAP - period. PP can also discuss her BC options with her - they will supply her with low cost or free BC. Pills are often low cost like $5 for a few months supply and condoms are free by the bag full, just need to ask for them.
Honestly, I would NOT recommend anything like DEPO, the IUD, longer pills like Seasonale, etc. I would also not recommend FAM - I WOULD get her the Taking Charge of Your Fertility book - as it is a wonderful wealth of knowledge about the reproductive systems, what is normal, what isn't, etc. She may get more reading about it than listening to someone talk to her about it. But I would not encourage her to try it now. Her cycles are too eratic still to use it as BC.
Personally if she was my kid (or one of my neices) I would recommend a low dose pill and condoms - with the emphasis on condoms. the pill as a "backup" - cuz a girl can never be too protected. But the primary BC should be condoms - really emphasis the STD issue.
If they only want to do condoms (one could only hope!) - then make sure she knows that emergency contraception is available and must be taken within 72 hours (and how it works - it is NOT an abortion pill). As a side note, do NOT allow her to get the new guardisil vax when she goes for her exam. I can forward you info on the severe reactions to it - including quite a few deaths - if you are interested.
also if they are monogamous and are intent on just using BC pills and not using condoms (like most teens are) - make sure they both go to PP and get tested for STDs first. This guy may not be her first, and at 19 I doubt she is his first. Also, remind her that even good relationships hit rocky points and cheating does happen. Condoms offer more protection - period.
as far as her dad - I think that she should be encouraged to talk to her dad about this - but I would not betray her trust and tell him. Maybe talk to him about the concept of the pregnancy test and her "friend" - to increase his awareness to his daughter and the relationship she is in. Especially considering the age difference - from experience I can say that most guys who are college aged and chasing after high schools girls - there is a reason for it, and the relationship may not be all that it seems.....high school girls are much easier to control and manipulate - especially if they are depressed or recovering from a major loss like this girl is....In addition to the sex and BC talk, I would really talk to her about the relationship and try to get to know her BF better - to make sure this is really a healthy relationship.