Your loved one is probably not consulting you to find out what you would do or what you think they should do. Even if they ask for your guidance, "I really don't know what you should do, but I'm here to listen if you need to talk through your thoughts" is the safest response. Expressing an opinion either way could come back to bite you. If you encourage separation, for example, and they later regret such a decision, you could find yourself blamed for their choice, whether this is fair or not. On the other hand, if you tell them the Bible says they have to try and work their marriage out because "that's what God would want," they may say you tried to make them feel guilty and judged for wanting to leave.
What you can do is encourage them not to make any major decisions while they are still so hurt and upset. Encourage them to wait until they're able to think a little more clearly and ot to take action impulsively or out of anger. "I know you're hurt and angry, and I'd probably be tempted to do the same thing, but I'm worried that you're going to regret doing or saying that," can be a way to address this without sounding judgmental or patronizing. Asking questions like "Are you sure you want to do that?" and "How is that going to help things?" might be good ideas if they're thinking about pulling a Carrie Underwood and vandalizing their partner's vehicle to teach him a lesson.
Many marriages survive affairs. Sometimes, the relationship is eventually better and stronger. "You two may be able to work this out" is a way to offer hope without it seeming like you are telling the person what they "should" do. Its probably not a good idea to tell them this may be a blessing in disguise, though. Plattitudes and nice sounding phrases don't help much either. "Everything will work out fine," or "God doesn't put more on you than you can handle," often shuts the other person down. Such cliches are sometimes what we hide behind when we don't know what to say but they ring very hollow to someone struggling with such a painful reality.
Remind them that there are counselors and clergy who can also help. Good people have affairs and they are more common than most people think. Your loved one is not the only person going through this and it is okay to remind them of that. The Beyond Affairs Network (BAN)
is a support group with meetings throughout the US. Your loved one may feel too overwhelmed and ashamed to consider anything like that at first, but just knowing groups like BAN exist may be a comfort early on.
There are lots of helpful books and articles on recovering from affairs. If you want to do some reading yourself to know what someone goes through after finding out, that may help. Letting them know you care enough to do some research could be very meaningful. "I've been reading this book I thought might be helpful to you if you'd like to hear about it," can be a gentle way to share what you're learning. If they decline, let them know they're welcome to ask you about it at a later time. Let them know you're just trying to know as much as you can, not so that you can influence their decisions or tell them how they're "supposed" to feel. There are lots of excellent resources on this topic at
Remember to take care of yourself and to cut yourself some slack.