If I marry someone who is Catholic, but wants to abort our child and has the abortion, am I also to blame for it? Part of my marriage vows are to support and ultimately stand by my wife, even when she makes a mistake (in my view). What should I have done?
It is not clear to me if this is a hypothetical question. If it is not a hypothetical question, please accept my sympathy on the loss of your child.
If a husband does all that he can to persuade his wife not to have an abortion but she does so anyway, he is not to blame for the abortion. In addition to seeking post-abortion grief counseling, which should be available from his diocese, I would recommend to such a husband that he seek out a balanced and orthodox confessor or spiritual director to discuss what steps he may licitly take to prevent another pregnancy, at least until such time as his wife repents for the abortion that she had. Although contraception is not allowed, it might be possible to insist either on strict NFP or complete abstinence from marital relations. A confessor or spiritual director should be consulted though before such steps are taken.
I also recommend marital counseling, which can be obtained from the Pastoral Solutions Institute. If the wife will not attend, the husband should seek such counseling on his own.