I’ve successfully raised two daughters, so I feel a little qualified to speak to this question.
To be honest, I don’t know that there is a lot that you can do. You are a friend, not even a relative. Your friend is a Protestant. He won’t comprehend why Mass is important. To him, it’s just another “worship service.”
Do you have the ear of the teenagers? Are you someone that they like or love and trust? If so, you can talk with them and explain the important of attending Mass. Tell them that the Mass is the source and summit of life for Catholics. It is of supreme importance. It’s not just another “church time” or “worship service.” It is where we receive JESUS, the Lord of heaven and earth. Tell them that you love them and you don’t want them to miss out on receiving Jesus.
But you have to be oh, so careful not to undermine the father’ work! The kids may play him against you if they sense that they can get something out of either of you.
You really have no right to interfere with his child-rearing unless he is abusing his teenagers.
You can be a good example to the teenagers no matter what.
Here is something to keep in mind: it is a quirk of human nature that when something is forbidden, that thing becomes unbearably attractive. But when permission is granted, the thing loses its luster.
Why else is it that when we aren’t dieting, we really don’t care about deep-fat fried chocolate-covered Peeps? But when we are on a diet, we crave these sickening treats like a junkie craves heroin, and we will go to elaborate lengths to “cheat” and obtain these forbidden pleasures.
I don’t think it would be the end of their Catholic faith if the teenagers are allowed to make the choice. If they are forbidden to attend Protestant worship services, those services will take on the appeal of a wondrous, magical, mysterious Disneyland to the teens. They will crave these services, they will long to go, and they will do anything, even something wrong, to be able to go.
But if they are given permission to attend Protestant worship services, eventually they will lose interest because they will discover that the services aren’t a wondrous, magical, mysterious Disneyland, but just a very long concert and lecture time.
Of course, this is assuming that the teenagers in question have been raised in the Church, have a well-formed conscience, and have received excellent catechesis about their Catholic faith.
If not, well–start praying hard that the Lord will have mercy. This is just the beginning. College will make mincemeat out of them, just as it has out of many kids. Only after they’ve been “out in the world” for several years will they come to their sense and realize the foolishness of rejecting God.
Good luck. It’s great that you care enough to try to help. Kids need all the advocates they can get.