Recommendations for Crossbow Optics?

I don't own a crossbow and really don't know much about them. A friend of ours wants to get a scope for her father for Fathers Day. He is getting on in years and is having trouble seeing his sights on his crossbow during low light conditions (sounds like me). I don't believe price is going to be a factor - she just wants to get him good quality optics. Any recommendations. Thanks in advance.