Any legal advice about an English girl marrying an American boy. Please.?

My daughter is currently staying with her fiance for 3 months at a time. She has a son of 3 years and it's stressful to keep leaving for a few weeks to comply with the 90 day law. They want to get married and start their own family very soon, but there seems to be a huge amount of red tape, and No certainty of the outcome.Has anyone got any advice for this situation. Thanks in anticipation.