Seeking advice from an older guy about a relationship my daughter is in

My Name is Nath, I'm 47 and my daughter is dating a much older guy and they're both very happy but for the longest time I've been trying to come to terms with it and understand from both her side and his, I'd appreciate any correspondence on how best for me to process this situation, the do's and don'ts etc as i'd like to be supportive, I just have concerns too. apologies if these goes against guidelines, thought this might be the place to ask