In my own experience, the best thing I can say is give your support. It’s not always an easy path, there are lots of decisions to make, considerations to think of and people will not always be supportive. Especially if they have a family who are against their vocation, just having someone who supports them that they can talk to is invaluable. If they want help researching orders, if they want company on retreats, as best as you can be there for them.
And let them share their vocation with you. My own vocation brings me such joy that I long to share it with others, but I know the people in my life are not ones who would share my joy. Discernment is not the proverbial ‘walk in the park’, it is difficult at times and it can be emotional. As I said, the best thing you can give is love and support.