Goodness, Mr. Ismi and I sat on this one for MONTHS, because we knew that no matter what we said, it wasn't going to end well. Both of his parents are charismatic pastors.
We ended up saying that although our priest knew that we were Christians, we had to partake in the sacraments under the Church to ensure that they were recognized by the Church -- all of them including marriage. We thought that it would be the best route to take because they think that my Roman Catholic baptism is "invalid," and I was badgered for years about getting baptized again. If they use the same rationale, they could understand the Orthodox standpoint.
They were INFURIATED. Baptism they could understand (and they even volunteered to come to it, even though they were afraid that we were joining a cult.
yeah....), but marriage made no sense to them whatsoever. They don't see it as a sacrament the way that the Orthodox do. We tried to explain it without saying, "Yeah, we NEED to get married in the Church."
My MIL asked Mr. Ismi if it was necessary and he kind of balked, so they thought that we could just do away with it. We had to tell them very firmly that we needed to and
wanted to, to have an Orthodox blessing of our marriage.
Basically, we had to word the issue in a way that was the least offensive, knowing that they would hate the reality of why we had to do the wedding service. And that did not work. I don't imagine that spelling it out would have been a better alternative.
I would just tell your friend to pray about it. Her parents may be skeptical about it. Conversion is never easy, even when you are an adult. We're still struggling with that, unfortunately.
I pray that they will find peace.