Need some advice on how others have coped when they don't necessarily care for their child's choice of significant other.

\\ - this is a new one for me as I have always liked and gotten along with my older sons girlfriends with no real issues or worries what so ever..... but the girl my youngest son (he is 19 yrs old) has picked for his present gf is to childlike in her behavior, words and actions and it is driving my husband and I crazy when she is over at the house with our son that still lives at home.

We have tried the approach of talking with our son - but in all honestly he can't control another person.
We have tried telling her how her behavior gets on our nerves at times - all the childish nick picking.
We have tried to educate then both on the fine art of relationships, but they still argue a lot - "covers ears"

I know she has been through a lot emotionally physically and medically with her having cancer (leukemia) for five years, and with this it has left her body not functioning as it should as a female or as a balanced peaceful person and yet my husband and I still find ourselves in the situation of not liking it when she is over visiting at our home.... my oldest son and his fiance feels the same, therefore, I know this is not just a case of mom and dad not liking who their children date.