Sam, if I said to you “don’t tell popa_bar_abba that I made his pot fleishig” would it be Lashon Hara for me to do so? There are situations where one must “violate” such requests and this to me sounds like one of them. This is his personal Rav who has been his dating coach, etc and now he has a serious “dating shayla”. Though perhaps in this situation, because the Rav is personally involved in the shayla, he should ask a different Rav.
He should probably first try to have an open conversation with his woman (that was for goq) about what exactly she doesn’t like about his Rav. If she doesn’t agree with some of the Rav’s hashkafos then they are going to have issues going forward in marriage as I assume this guy wants to run his home based on his rebbe’s hashkafos. As has been discussed elsewhere in the CR, in general couples will follow the husband’s halachos. The guy here should explain to his potential kalla that when it comes to halacha, you can’t just pick and choose rabbanim – if you have a Rav who you have always followed, you should continue doing so unless there is a very good reason to switch such as the Rav ch”v going off the derech. If the wife isn’t going to respect her husband’s Rav’s opinion then the husband is going to have a hard time getting her to follow all the halachik of this Rav.
I don’t think “hakaras hatov” is necessarily the issue here. It sounds like the Rav has been very helpful to the couple but for whatever reason the young lady felt that he was rude or something and is uncomfortable with him. It is very possible that this was a simple misunderstanding between the girl and the Rav. Which is why the guy needs to speak to her to find out specifically what bothered her and then needs to speak to a Rav (possibly this one, possibly another) to see whether her complaints are valid.