First of all you need to determine if she wants your advice. Is she asking for your opinion, or for biblical guidance? If she is, great. If she's not, she's probably not going to hear anything you have to say anyway.

In Matthew 7, Jesus advises us to not give what is holy to dogs, or throw our pearls before swine. There is a time for advice and counsel, but if a person doesn't WANT it, it's unlikely she's going to hear even the best advice.

BUT, if you're good friends and you really care about her, you're probably going to try anyway, which is fine. Try once, maybe twice, but if she continues to ignore/refuse you - stop. It's not the time for advice. Pray for her, love her, be an example, and when Holy Spirit has done His work on her heart she'll come to you because she knows she can, and then she'll be ready to hear you. (Or, when the relationship falls apart - as it very likely will - she'll know she can come to you for comfort and you won't rebuke her with "I told you so"s.)

We discourage dating - "missionary" or not - for five reasons. Maybe some of this will help.

1 - It's not Biblical. Scripture doesn't discuss dating, because it didn't happen. Scripture does, however, discuss guarding our hearts (Prov 4:23) and seasons in life (Ecc 3). Anyone living at home and going to school is in a season of preparation - study, learn, embark on a career, define YOURSELF, etc. No one can be in a healthy relationship until she knows who she is in Christ, and the calling God has on her life first.

2 - It profits nothing. Young people who date do not encourage each other in their faith like they always say it will. It's just a distraction. Paul said all things are lawful (scripture never says, "Thou shall not date"), but not all things are helpful. (1 Cor 6:12) That means yes, you can, but are you trying to get away with as much as you can, or are you trying to draw as close to God as you can? Is "okay" your standard, or is "Christlike" your standard?

"Missionary" dating is a lie that comes in here. It doesn't work, and to say that it's necessary puts a very short leash on the Holy Spirit. He doesn't need you to help Him save someone.

3 - It leads to temptation. At best, dating is just distracting. At worst, it leads young people into physical and emotional relationships that create a further distance between them and God.

4 - It ends. There are romantic stories about couples who meet in high school, but they are the RARE exception. They usually end - usually when college starts and your world blows wide open - and they end in heartache and fractured soul ties and emotional baggage and regrets.

5 - It's unnecessary. Some like to say that we have to date if we're going to find a spouse. God knows who would make the best husband for your friend. If she really wants the best she can get, it would make more sense to NOT date, NOT bias herself, and spend this season of her life drawing close to God and learning to hear His voice. That way, when the time comes, she can hear God when he says, "That one."