Dear 100 Hour Board,
So, what advice would you give someone who feels like they made a mistake marrying someone? They say they had a choice between two people and they chose the wrong person.
I would definitely advise a marriage counselor. But as Anne says below, for such a sensitive topic, only offer advice that has already been solicited. This is the kind of issue that merits professional help, in my opinion.
I'm not sure how to package what I'm going to say next as advice for your friend, but I would like to include it in this answer because it's pertinent to the subject: I don't believe that outside of situations along the same line as abuse there is really a wrong choice for who we marry. It's not so much much who we initially choose, but the choices we make thereafter that truly determine how much of a match someone is. This is because people are dynamic; who you marry is going to be a different person from the person you celebrate your 25th anniversary with. Thus it's the life you create together that truly makes a difference.
Anathema's recommendation to seek professional advice is good. I'd also be cautious of offering advice in this situation if the person wasn't looking for it. Particularly in a very sensitive situation like this, there are a few potential advice-giving pitfalls:
1) Not knowing the whole situation. Do you know all the details about a) why she chose Guy A instead of Guy B and b) why she now feels that the choice was wrong and c) what the Guys or others may have done to contribute to those feelings and d) etc. etc. etc. If the answer to the above is "no," you run the risk of giving advice that could be not just useless but actively harmful. (For example, imagine if it turns out that her now-Husband is abusive and you don't know. You certainly wouldn't want to go on to give advice about "Well, maybe if you just give it some time...")
2) Not knowing if the person actually wants advice. A lot of the time we go to our friends for validation of our feelings ("yeah, I felt like that once" or "ugh, I'm sorry...") rather than for solutions. It's the whole "not about the nail" thing (which both genders are actually prone to, though it may well be imbalanced the way the video suggests). Especially on a matter as a) personal and b) important as a marriage that already took place, I'd usually want to be pretty sure the person actually wanted my suggestions before I offered them.
If someone said "Anne, hypothetically what if someone had been dating two worthy, righteous priesthood holders and then married one of them and despite having a healthy and supportive relationship felt that maybe they had chosen wrong, what would you think about that?" then my thoughts would probably be along the lines of the comments President Monson made in this talk regarding making a marriage work. However, I think you'd need to be really really careful to make sure that any such advice was both a) appropriate for the person's situation and b) appropriate for you specifically to offer.