I really wouldn’t worry.
Kids of 20 are forever falling in and out of love! You say “ lucky they have n children yet” - did you mean your son and his girlfriend? I should hope not, it sounds very much like puppy love.
How, out of interest , does she prevent contact other than by text?
It might that he is just infatuated but it could also be a red flag that she is of a controlling nature but he is too young and inexperienced to realise.
What do other people who know her think?
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