If two people are married in the Catholic Church and 25 years later, the wife decides that she wants to become a Protestant, is it a mortal sin for the husband to remain married to her if she will not recant that decision? Does the marriage become invalid?

How on Earth could honoring one's marriage vows possibly be a SIN?

Well, that was my thought, too.

If a marriage is invalid, it is invalid at the time it was attempted. Nothing that happens later can invalidate it. Sometimes things happen later that can serve as evidence that it was invalid, but they themselves don't make it invalid. In this case, there is a bond between the two that they cannot break, and if the marriage was consummated (a fairly safe bet after that much time), nothing but death can break the bond.

1. Consumated marriage can only be dissolved by death. Either it has always been a marriage, or it never was. Later acts might bring to attention things that were present from the beginning, but have no bear in themselves on validity

2. If her defection of faith makes it morally impossible to continue common life, separation is justified. But it must be a truly grave reason, not just because you do not want to deal with it. For example, if you have children, her heresy and schism could be a danger to their faith.

But, unless you are in one of a few countries where the Church court handles separation, and civil effects are obtained, a separation will require civil court. If the only cause is her wanting to take tye children from the Church, a separation could just likely give her primary custody. It would be best to do all one can to keep common life and raise the children Catholic, but she might oppose that. If she acts in a way to make it impossible, then separation might be the call, but it will be unfortunate