@Scribe
My concerns would be for the values by which he makes decisions and his attitude of the heart. What is the attraction beyond physical? What do they have in common? How can her emotional growth and understanding be at a place where she would be a wise companion? I couldn't imagine what I'd have to say to a 20 year old. Also, they are in very different stages of life. What are her dreams? Does she want children? She might say it doesn't matter in order to live out a romantic fantasy, but at that point she might not know her heart. Does she realize she will only be 35 when her husband qualifies for a pension and a cheap dinner at Denny's?
As a member of the congregation I would be disappointed that my pastor was so short sighted in terms of what is needed to make a marriage work. I would dislike the idea that other men in the congregation think it is OK to go after younger women, like the 48 yr old making advances to an 18 yr old girl. I would want a pastor with better values and who would find someone who could be a complete companion. The young lady needs the chance to have a family with someone who will be around, have the energy and fit in with her friendship group. Since you asked for an assessment, I would think he had poor judgement and was selfish when it came to her long term needs. Could you imagine mentioning that you were researching the Diet of Worms (1521AD) and have her asked if that was a bird food mixture? Just saying.